A composition suitable for producing detergent foams of high stability in aqueous systems, comprises (1) an organic surfactant, and (2) an organosiloxane-oxyalkylene block copolymer in which the siloxane and oxyalkylene blocks are interconnected by a divalent hydrocarbon radical, the weight ratio of the copolymer to the surfactant being from 0.001:1 to 99:1 (see Division C3).  The block copolymers are described in Specifications 916,561 and 1,034,781. The surfactants disclosed are anionic, catronic, non-ionic and amphoteric, and numerous examples thereof are given. The compositions may also contain builders to increase the surfactant activity, e.g. alkali metal carbonates, phosphates, borates and silicates; sodium sulphate and chloride; water-soluble gums, starches and proteins; sodium carboxymethyl-, methyl- and hydroxymethyl-celluloses; sodium polyacrylate; urea and thiourea; sodium citrate and lactate; fatty acid amides, nitriles and alkylolamides; and morpholides.  In the example, several portions of an aqueous solution of sodium lauryl sulphate modified with different amounts of a siloxane-oxyalkylene block copolymer, were diluted and the surface tension of each portion was measured.ALSO:A composition suitable for use in reducing the surface tension of aqueous solutions comprises (1) an organic surfactant and (2) an organosiloxane-oxyalkylene block copolymer having an affinity for water and comprising (a) at least one siloxane block containing at least 2 units of the formula <FORM:1034782/C3/1> wherein R is a monovalent hydrocarbon (saturated or unsaturated) or halo-hydrocarbon radical or a divalent hydrocarbon radical and b is 1, 2 or 3, the block containing at least one siloxane unit in which R is a divalent hydrocarbon radical, and (b) at least one oxyalkylene block containing at least 2 -R1O- units, wherein R1 is an alkylene radical, the siloxane and oxyalkylene blocks being interconnected by the divalent hydrocarbon radical, the weight ratio of the copolymer to organic surfactant being from 0.001:1 to 99:1.  The siloxane block may contain terminal hydroxy, alkoxy, aryloxy or acyloxy radicals and may also contain <FORM:1034782/C3/2> units, wherein e is 0, 1 or 2, f is 1 or 2 and e + f is 1, 2 or 3. The block copolymers are described in Specifications 916,561 and 1,034,781.  The surfactants disclosed are anionic, cationic, nonionic and amphoteric and numerous examples thereof are given. The compositions may also contain "builders" to increase the surfactant activity, e.g. alkali metal carbonates, phosphates, borates and silicates; sodium sulphate and chloride; water-soluble gums, starches and proteins; sodium carboxymethyl-, methyl- and hydroxymethyl - celluloses; sodium polyacrylate; urea and thiourea; sodium citrate and lactate; fatty acid amides, nitriles and alkylolamides; and morpholides.  In the example, several portions of an aqueous solution of sodium lauryl sulphate modified with different amounts of a siloxane-oxyalkylene block copolymer, were diluted and the surface tension of each portion was measured.  Uses.-As detergents and scouring agents, car washes, rug and hair shampoos, bubble baths, upholstery cleaners, cosmetics, bottle washers, dentifrices and shaving soaps; for making foams useful to entrain air in concrete and cinder block mixes; as flotation agents in metal ore separation and recovery systems; and as a dye assistant, textile dye leveller or dye dispersant.