Hydrophobic organosilicon powders, having bulk densities of less than 0.3 g. per c.c. and useful for thermal insulation, flatting agents in paints, cosmetic bases and reinforcing fillers for siloxane elastomers, are made by adding (1) an organosiloxane of the average general formula RnSiO 4-n/2 , in which each R is an aliphatic hydrocarbon radical containing less than 5 carbon atoms or a phenyl radical, there being not more than one phenyl radical attached to each Si atom and n has a value from 2 to 3 inclusive, to (2) a silica organogel, the organogel (2) having a density of from 0.02 to 0.35 g. of total SiO2 per c.c., the siloxane (1) being present in such an amount that there is at least 0.04 organosilyl units of compound (1) per SiO2 unit of compound (2), in the presence of a strong acid catalyst, and thereafter removing the volatile materials from the reaction product thus formed.  The silica organogel (2) is preferably made by acidifying a sodium silicate solution, allowing the silica to gel and thereafter replacing the water with an organic solvent, e.g. an aliphatic or aromatic hydrocarbon, a chlorinated hydrocarbon, a ketone, an ether, a tertiary amine or a low viscosity siloxane. Organic solvents may be used for reacting siloxane (1) with the organogel (2).  The siloxane may serve both as the solvent in the organogel and as reactant (1).  Thus the compounds of the invention may be made by adding the siloxane directly to a hydrogel having the composition of compound (2), and the reaction therebetween may be effected at a temperature from 30 DEG  C. to the reflux temperature of the solvent.  Catalysts specified are chlorosilane, hydrochloric acid, sulphuric acid and benzene sulphonic acid.  Examples 1 to 3 disclose the preparation of powders from silica hydrogels, siloxanes and solvents. Specifications 682,540, 682,574 and 783,867, [Group IV (a), are referred to.ALSO:Hydrophobic organosilicon powders, having bulk densities of less than 0.3 g. per cc. and useful for thermal insulation, flatting agents in paints, cosmetic bases and reinforcing fillers for siloxane elastomers, are made by adding (1) an organosiloxane of the average general formula RnSiO 4-n/2 in which each R is an aliphatic hydro carbon radical containing less than 5 carbon atoms or a phenyl radical, there being not more than one phenyl radical attached to each Si atom and n has a value from 2 to 3 inclusive; to (2) an organogel in which at least 50 mol. per cent of the polymer units are SiO2 units, any remaining units being of the general formula R1SiO1.5 in which R1 is an aliphatic hydro carbon radical containing less than 5 carbon atoms, the organogel (2) having a density of from 0.02 to 0.35 g. of total SiO2 and R1SiO1.5 per cc., the siloxane (1) being present in such an amount that there is at least 0.04 organosilyl units of compound (1) per polymer unit of compound (2), in the presence of a strong acid catalyst, and thereafter removing the volatile materials from the reaction product thus formed.  The organogel (2) may be a silica organogel or a cogel of silica and a mono-organosiloxane, e.g. mono-methyl-, propyl-, vinyl-, allyl- or butyl-siloxane, the former preferably being made by acidifying a sodium silicate solution, allowing the silica to gel and thereafter replacing the water with an organic solvent, e.g. an aliphatic or aromatic hydrocarbon, a chlorinated hydrocarbon, a ketone, an ether, a tertiary amine or a low viscosity siloxane, and the latter best being made by acidifying a solution of a mixture of sodium silicate and a salt of the general formula R1Si(ONa)xO 3-x/2 , wherein x has a value from 1 to 2, the water in the co-gel being subsequently replaced by an organic solvent.  Organic solvents may be used for reacting siloxane (1) with the organogel (2).  The siloxane may serve both as the solvent in the organogel and as reactant (1). Thus the compounds of the invention may be made by adding the siloxane directly to a hydrogel having the composition of compound (2), and the reaction therebetween may be effected at a temperature from 30 DEG  C. to the reflux temperature of the solvent.  Catalysts specified are chlorosilane, hydrochloric acid, sulphuric acid and benzene sulphonic acid.  Examples 1 to 3 disclose the preparation of powders from silica hydrogels, siloxanes and solvents, and Examples (4) to (8) disclose the preparation from cogels, siloxanes and solvents.  30 parts by weight of the powder of Example (1) were milled with 100 pts. of a dimethylpolysiloxane gum and 3 pts. of t-butyl perbenzoate, and the mixture was heated in a mould at 150 DEG C. to give an elastomer.  Specifications 682,540, 682,574 and 783,867 are referred to.