SiCO glasses prepared from sol-gel precursors via pyrolysis in argon at temperatures ranging from 1000 degrees to 1400 degreesC were studied by transmission electron microscopy (TEM), in conjunction with electron energy-loss spectroscopy (EELS). EELS analysis showed that stoichiometric SiCO glass underwent phase separation, forming SiO2- and SiC-based environments. This process started at similar to 1200 degreesC. However, at temperatures > 1300 degreesC, precipitation of nanometer-sized SiC particles embedded in vitreous SiO2 was monitored by high-resolution TEM.