Gels have been prepared via hydrolysis–condensation reactions of triethoxysilane (TREOS) and methyldiethoxysilane (MDES) in various ratios. The hydrolysis–condensation reactions of the precursors have been investigated by ²⁹Si and ¹⁷O solution NMR. Both precursors present a very high reactivity towards hydrolysis and condensation reactions, and formation of co-condensed species will be discussed. These highly reactive precursors lead to transparent gels over a large composition range, 0.1 ⩽ MDES:TREOS ⩽ 10. Their structure was investigated by ²⁹Si MAS NMR, FTIR and DSC: the gels appear as highly condensed homogeneous systems in which the difunctional and trifunctional units are randomly distributed. The microstructure of the gels was also investigated through the measurement of specific surface area and skeletal density. Finally, some physical properties have also been measured such as thermal stability in O₂ and inert atmosphere (Ar), coefficient of thermal expansion (α) and elastic modulus (E).