Silicon oxycarbide glasses (Si/C/O) of various compositions have been obtained after pyrolysis of polysiloxane gels produced by the sol-gel method. Four gels were synthesized from various structures of silicon precursors: MTES issued from methyltriethoxysilane CH3Si(-OEt)(3), VTES from vinyltriethoxysilane CH2=CH-Si(-OEt)(3), PTES from phenyltriethoxysilane C6H5-Si(-OEt)(3) and (DTH)-T-H 1/9 for that issued from the 1/9 molar ratio of methyldiethoxysilane D-H (CH3)HSi-(OEt)(2) and T-H triethoxysilane HSi(-OEt)(3). The resulting materials at 1000 degrees C can be described as an oxycarbide phase with the presence of either an excess of carbon or silicon depending on the starting structure of the precursor used to prepare the gels. By thermogravimetric analysis (TGA) coupled with mass spectrometry (MS), we followed the thermal behavior of each compound from 1000 degrees to 1500 degrees C. Based on these results and chemical analysis data, we have established the main reactions that occur during the decomposition of the oxycarbide glasses. At low temperature (T < 1400 degrees C), the dominant mechanism for carbon-rich materials first involves a solid-state reaction of SiO2 and C leading to SiC formation and removal of CO. That reaction proceeds at high temperature (above 1400 degrees C), as long as the amount of carbon in the material is high enough (PTES). When the system is depleted of its free carbon (VTES, MTES), however, another reaction can also proceed parallel to the former one, where SiO2 and SiC react leading to a loss of SiO and CO. The combination of an excess of silicon and a dense state of the material improves the thermo-chemical stability of the silicon oxycarbide phase present in the (DTH)-T-H 1/9 glass. By placing the chemical compositions in the ternary Si-C-O diagram, we could then determine the evolution at high temperature of any system of mixed Si/C-x/O-y phases.