Crystallization behavior of Si-C-O glasses in the temperature range of 1000 degrees -1400 degreesC was investigated using transmission electron microscopy (TEM) in conjunction with electron energy-loss spectroscopy (EELS). Si-C-O glasses were prepared by pyrolysis of polysiloxane networks obtained from homogeneous mixtures of triethoxysilane, T-H, and methyl-diethoxysilane, D-H. Si-C-O glass composition depended on the molar ratio of the precursors utilized. At a ratio of T-H/D-H = 1, the formation of a carbon-rich glass was observed, whereas a ratio of T-H/D-H = 9 yielded a Si-C-O glass with excess free silicon. Both materials were amorphous at 1000 degreesC, but showed a distinct difference in crystallization behavior on annealing at high temperature. Although T-H/D-H = I revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400 degreesC, T-H/D-H = 9 showed, in addition to SiC crystallites, numerous larger silicon precipitates (20-50 nm), even at 1200 degreesC. Both materials underwent a phase separation process, SiCxO2(1-x) --> xSiC + (1-x)SiO2, when annealed at temperatures exceeding 1200 degreesC.