Methyl-, vinyl- and octadecylsilsesquioxane gels have been prepared by the solvent-free nonhydrolytic condensation of organotrichlorosilane with oxygen donors in the presence of a catalytic amount of various metal chlorides. Pure phenylsilsesquioxane gels could not be obtained by this route owing to the partial ipso-cleavage of phenylsilicon bonds by the alkyl chloride formed during the condensation process. The rate of condensation depends on the nature of both the oxygen donor and the catalyst. For highly condensed methylsilsesquioxane gels, the extent of silicon loss by Si–O/Si–O and Si–C/Si–O redistribution reactions during pyrolysis depends mainly on the nature of the oxygen donor, whilst the composition of the volatile silicon species, and consequently that of the residue, depends on the nature of the metal chloride initially introduced. This process allows the formation of a rather well-condensed octadecylsilsesquioxane gel which is constituted of small particles assembled by crystallization of the long C18 chain.