Copolymerization of tetraethoxysilane (TEOS) and dimethyl(diethoxy)silane (DMDEOS) was studied by means of the Si-29 NMR spectroscopy and ab initio quantum chemical calculations. Si-29 NMR spectra of reaction mixtures of TEOS and DMDEOS were effectively measured by a slightly modified DEFT experiment, in which the pulse sequence is divided into two parts and all pulses on protons are replaced by selective pulses. Quantum chemical ab initio calculation of structures and Si-29 NMR chemical shifts of some reaction products were used in signal assignment. The assignment of the signals to corresponding structure units is not yet unambiguous; however, a semiquantitative analysis of relations in the reaction mixture of TEOS and DMDEOS could be done. Although the reactivity of the DMDEOS monomer is much higher in comparison with TEOS, the arising gel is not strictly phase separated and copolymerization of both monomers occurs. In the first stages of the process, resulting oligomers are composed of 70% of DMDEOS structure units. In the final stages of polycondensation, domains of the TEOS structure units are formed, which can act as linking units between cyclic oligomers.