29Si and 13C MAS NMR spectroscopy, DTA-TG and elemental analysis were used to study the evolution of metastable oxycarbide phases starting from thin gel sheets derived from methyltriethoxysilane (MTEOS), tetraethoxysilane (TEOS) and a colloidal silica containing sol, up to 1450°C under a nitrogen atmosphere. The stoichiometric formulae of the glasses obtained by the semi-quantitative analysis of 29Si MAS NMR spectra were found to be in good agreement with those obtained by chemical analysis. It was found that the addition of colloidal silica sol delays the decomposition of methyl groups in MTEOS containing condensates. This delayed decomposition of the methyl groups was attributed to facilitate the strengthening of the matrix, thus leading to crack free sintering of sheets up to 400°C in air and for T>900°C under nitrogen. A significant portion (>40%) of the total carbon present in the glasses was found to exist as free carbon. 13C NMR showed that unsaturated C=C bonds exist in gels heat treated at 1000 °C, but they disappear in heat treated gels at 1200 and 1450°C. Both 29Si and 13C NMR spectra indicate the formation of a ca. 15 atom% SiC phase in the glass sheets sintered at 1450°C, which were found to be amorphous up to 1200°C.