A series of silsesquioxane copolymers synthesized by hydrolysis and condensation of phenyl- and methyltrimethoxysilanes have been studied as preceramic polymers. The pyrolytic conversion to ceramics was characterized by thermogravimetric analysis, Si-29 and C-13 nuclear magnetic resonance and Raman spectroscopy. The pyrolysed materials were further characterized by differential thermal analysis, X-ray diffractometry and transmission electron microscopy. The ratio of phenyl to methyl groups in the copolymer was found to control polymer structure and rheology, as well as ceramic composition and char yield. On pyrolysis to 1000-degrees-C under inert conditions, silicon oxycarbides were formed, along with glassy carbon. On heating from 1200-degrees-C to 1400-degrees-C, the oxycarbide structure diminished, and the materials were comprised primarily of amorphous silica, amorphous Si-C, some small crystallite SiC and graphitic carbon. The carbon content increased, and char yield decreased, with increasing concentration of phenyl groups in the copolymer. The presence of free carbon appears to inhibit the crystallization of silica. Significant carbothermal reduction was observed only above 1500-degrees-C. Oxidation studies of the pyrolysed materials indicated the presence of at least two forms of carbon.