1.3.4-Oxdiazoles, free from hydroxyl groups and acid groups imparting solubility in water, and substituted in each of the 2- and 5- positions by a residue of the benzene series, at least one of these residues being substituted by at least one aliphatic hydrocarbon group containing at least 4 carbon atoms, are used as light-screening agents for incorporating in various substrata to protect them from ultra-violet light or for incorporation in films which are used to protect the substrata.  Among the substances which may be so protected are oils, fats and waxes, such as edible oils and fats; candles; floor polishes; floor stains or other wood stains; furniture polishes; dyed or natural soaps; and shampoos. Specification 816,750 is referred to.ALSO:1,3,4-Oxdiazoles, free from hydroxyl groups and acid groups imparting solubility in water, and substituted in each of the 2- and 5-positions by a residue of the benzene series, at least one of these residues being substituted by at least one aliphatic hydrocarbon group containing at least 4 carbon atoms, are used as light-screening agents for incorporating in various substrata to protect them from ultra-violet light or for incorporation in films which are used to protect substrata from ultra-violet light. They may be added, for example, to the spinning mass of acetylcellulose (including cellulose triacetate) or to the mass from which fully synthetic fibres are made, e.g. polyamide melts or polyacrylonitrile spinning masses, prior to, in the course of, or subsequently to the polycondensation or polymerization respectively.  They may also be added to lacquers and films, e.g. those made from acetyl cellulose, cellulose propionate, cellulose butyrate, nitrocellulose, vinyl acetate, polyvinyl chloride, copolymers of vinyl chloride and vinylidene chloride, alkyd lacquers, polyethylene, polyamides, polyacrylonitrile, polyesters and in transparent foils of regenerated cellulose or acetylcellulose.  They may also be added to natural or synthetic resins, e.g. epoxy resins, polyester resins, vinyl resins, polystyrene resins, alkyd resins, phenol-, urea- and melaminealdehyde resins, emulsions of synthetic resins, synthetic resins reinforced with glass fibres and natural or synthetic rubber-like materials such as rubber, balata gum, gutta-percha, polychloroprene, olefinic polysulphides, polybutadiene, butadiene-styrene copolymers and butadiene-acrylonitrile copolymers.  Specification 816,750 is referred to.ALSO:The invention comprises 1,3,4-oxdiazoles which are free from hydroxyl groups and acid groups imparting solubility in water and are substituted in each of the 2- and 5-positions by a residue of the benzene series and contain at least one aliphatic hydrocarbon residue containing at least 4 carbon atoms.  They may be prepared by treating a diacylhydrazine of formula <FORM:0890461/IV (b)/1> in which R1 and R2 each represent a residue of the benzene series at least one being substituted by an aliphatic hydrocarbon residue having at least 4 carbon atoms, with a nonsulphonating agent capable of eliminating water.  The diacyl hydrazines used as starting materials may be obtained by condensing hydrazine at both nitrogen atoms, or a monoacyl hydrazine of formula <FORM:0890461/IV (b)/2> at the -NH2 groups, with an acid halide of formula <FORM:0890461/IV (b)/3> The residues R1 and R2 may contain several of the aliphatic residues containing at least 4 carbon atoms.  These aliphatic residues may be branched or unbranched.  The preferred diacylhydrazines for use as starting materials are those of formula <FORM:0890461/IV (b)/4> in which R is an aliphatic hydrocarbon radical containing at least 4 carbon atoms, A is hydrogen or an alkoxy group containing not more than 3 carbon atoms and B is hydrogen, a chlorine atom, or an alkyl group, e.g. methyl or tertiary butyl, an alkoxy group containing not more than 3 carbon atoms or a phenyl residue. Non-sulphonating agents capable of eliminating water from the diacyl hydrazines are, for example, anhydrous aluminium chloride, phosgene, phosphorus trichloride, pentachloride or oxychloride, and, preferably, thionyl chloride.  The dehydration may be carried out by heating the diacylhydrazine with the water-eliminating agent in an inert organic solvent.  The new compounds may be used as light-screening agents for various materials.  Examples are given of the preparation of compounds of formula <FORM:0890461/IV (b)/5> and <FORM:0890461/IV (b)/6> Specification 816,750 is referred to.ALSO:1,3,4-Oxdiazoles, free from hydroxyl groups and acid groups imparting solubility in water, and substituted in each of the 2- and 5-positions by a residue of the benzene series at least one of such residues being substituted by at least one aliphatic hydrocarbon group containing at least 4 carbon atoms, are used as light-screening agents for a variety of materials. These compounds may be incorporated with the substratum to protect it from attacks by ultra-violet rays, by way of a dyeing process; or they may be incorporated with a "filter coating" to protect a substratum placed immediately underneath the filter or some distance from it.  Materials that can be protected are textile materials in general, e.g. fibres, filaments, yarns, woven or knitted fabrics or felt and finished products made therefrom, consisting of natural materials of animal or vegetable origin, semi-synthetic materials such as regenerated cellulose, rayon or spun rayon, or synthetic materials obtained by polymerization or copolymerization.  The protective compounds may also be added to coating and dressing agents for textiles, e.g. those based on starches, casein or synthetic resins made from vinyl acetate or derivatives of acrylic acid. Specification 816,750 is referred to.ALSO:1.3.4-Oxdiazoles, free from hydroxyl groups and acid groups imparting solubility in water, and substituted in each of the 2- and 5-positions by a residue of the benzene series, at least one of these residues being substituted by at least one aliphatic hydrocarbon group containing at least 4 carbon atoms, are used as light-screening agents for incorporating in various substrata to protect them from ultra-violet light or for incorporation in films which are used to protect substrata from ultra-violet light. They may be used for the protection of butter, feathers, hairs, pelts, hides, straw and wood, for example, or for incorporation in cosmetics such as perfumes, bath salts, skin and face creams, lipsticks, powder, brilliantines, hair sprays, insect repellents and sunburn lotions and oils.  Examples are given of the preparation of sunburn preparations and insect repellents containing one of the substituted 1.3.4-oxdiazoles defined above with (a) adipic acid isopropyltetrahydrofurfuryl ester and glycerol monostearate in alcoholic solution; (b) trichloromonofluoromethane, dichlorodifluoromethane, ethanol and perfume oil; (c) adipic acid isopropyltetrahydrofurfuryl ester, benzoic acid diethyl amide and dimethyl phthalate dissolved in peanut oil and paraffin oil; (d) glycerol monostearate, cetyl alcohol, sodium cetyl sulphate, stearic acid and glycerol; and (e) adipic acid isopropyltetrahydrofurfuryl ester, toluic acid diethylamide, cetyl alcohol, petroleum oil, beeswax, lanolin, cocoa butter, water and sodium benzoate. Specification 816,750 is referred to.