1:3:4 Oxadiazoles of the formula &lt;FORM:0881537/III/1&gt; in which X represents a hydrogen atom or an alkoxy group of 1 to 3 carbon atoms or a phenyl residue, are used as light screening agents for incorporation with hydrophobic substances containing oils, fats or waxes, such as edible oils or fats, candles, floor polishes, floor stains or other wood stains, or furniture polishes.ALSO:1:3:4-oxadiazoles of the formula &lt;FORM:0881537/IV (a)/1&gt; in which X represents a hydrogen atom or an alkyl or alkoxy group of 1 to 3 carbon atoms or a phenyl residue, are used as light-screening agents for incorporation with organic materials. Organic materials specified include textile materials of semi-synthetic or synthetic type, such as regenerated cellulose, e.g. viscose rayon, polyacrylonitrile, polyesters and polyamides; lacquers and films, e.g. of acetyl cellulose, cellulose propionate, cellulose butyrate and mixed cellulose esters, nitro cellulose, vinyl acetate, polyvinyl chloride, copolymers of vinyl chloride and vinylidene chloride, alkyd resins, polyethylene, polyamides, polyacrylonitrile, and polyesters; natural and synthetic resins, e.g. epoxy resins, polyester resins, vinyl resins, polystyrene resins, alkyd resins and aldehyde resins, such as condensation products of formaldehyde with phenol, urea or melamine, and emulsions of synthetic resins; synthetic resins reinforced with glass fibres and laminates made therefrom; rubber, balata, and gutta-percha; synthetic vulcanizable materials such as polychloroprene, olefinic polysulphides, polybutadiene, and copolymers of butadiene with styrene or acrylonitrile.  The rubber-like materials may contain fillers, pigments, vulcanization accelerators.  The light-screening agents may be added to spinning masses, e.g. polyamide melts or polyacrylonitrile spinning masses, and may be added before, during or after the polymerization.ALSO:The invention comprises 1 : 3 : 4-oxadiazoles of the formula &lt;FORM:0881537/IV (b)/1&gt; in which X represents a hydrogen atom or a lower alkyl or alkoxy group containing not more than 3 carbon atoms, or a phenyl residue.  The novel compounds may be prepared by treating a diacyl hydrazine of the formula Cl.C6H4.CO.NH.NHCOC6H4X with a non-sulphonating agent capable of eliminating water, e.g. anhydrous aluminium chloride, phosgene, phosphorus oxychloride or penta- or tri-chloride or thionyl chloride. The diacyl-hydrazines may be obtained by condensing a monoacyl-hydrazine R1CONHNH2 with an acid halide Halogen-CO.R2 where one of the residues R1 and R2 has the formula ClC6H4- and the other X.C6H4-, advantageously in an organic solvent and in the presence of an acid-binding agent e.g., pyridine or picoline. The monoacylhydrazines are obtained by acylating hydrazine with an ester or acid halide of R1COOH.  The novel compounds may be used as light-screening agents.ALSO:1 : 3 : 4-oxadiazoles of the formula &lt;FORM:0881537/IV (c)/1&gt; in which X represents a hydrogen atom or an alkyl or alkoxy group containing 1 to 3 carbon atoms or a phenyl residue, are used as light-screening agents for incorporation with textile materials.  Materials specified are wool, silk, cotton, hemp, flax, linen, jute and ramie; semi-synthetic materials such as viscose rayon and synthetic materials such as polyacrylonitrile, polyesters and polyamides.  The materials may be in the form of fibres, filaments, yarns, woven or knitted fabrics or felt, or the finished products made therefrom.  The compounds may also be incorporated in coating and dressing agents for textiles, for example those based on starches or casein or on synthetic resins, for example, those obtained from vinyl acetate or derivatives of acrylic acid.ALSO:A preparation for affording protection against ultraviolet rays comprises a 1 : 3 : 4-oxadiazole of the formula &lt;FORM:0881537/VI/1&gt; in which X represents a hydrogen atom or an alkyl or alkoxy group containing 1 to 3 carbon atoms or a phenyl residue, in admixture or conjunction with a carrier material.  The oxadiazoles may be employed in cosmetics, such as perfumes, dyed or natural soaps and bath salts, shampoos, skin and face creams, lipsticks, powder, brilliantines, hair sprays, insect repellents and more especially sunburn oils and creams.  The 1 : 3 : 4 oxadiazoles may also be used as light screening agents to protect edible oils, fats or butter by incorporation therewith.