PMMA–polysilsesquioxane (PMMA–Sil) class II organic–inorganic hybrids were prepared by the sol–gel method from a PMMA-based polymer precursor containing trimethoxysilane groups. An analysis was made of the effect of siloxane content, adjusted by addition of tetraethyl orthosilicate (TEOS), on the structure and thermal stability of the dried gels. ¹³C nuclear magnetic resonance measurements confirmed PMMA as the organic phase, while ²⁹Si measurements revealed the presence of both T and Q silicon species, the most abundant being T² and Q³. X-ray diffraction results showed that the inorganic SiO₂ phase was amorphous, while small angle X-ray scattering analyses indicated that the average gyration radius size of the silicate particles and the correlation distance between the particles increased with greater addition of TEOS. Thermal stability was improved by increasing the amount of the inorganic phase. This effect was more evident under an air atmosphere than under N₂, indicating that the silicate phase acted to limit oxygen diffusion.