Transparent monolithic gels were prepared from the cohydrolysis of diethylphosphatoethyltriethoxysilane (OC2H5)(2)P(O)CH2CH2Si(OC2H5)(3) (SiP) and tetraethoxysilane Si(OC2H5)(4)(TEOS) in various ratios, under acidic conditions. The hydrolysis and initial polycondensation of SIP were studied by high-resolution Si-29-NMR. Based on signal intensities, signals due to the following oligomers were identified : R(OH)(2)SiOSi(OH)(R)OSi(OH)(2), R(OH)(2)SiOSi(OH)(2), R(OH)(OEt)SiOSi(OH)(2)R, R(OH)(OEt)SiOSi(OH)(2)R, R(OH)(2)SiOSi(OH)(R)OSi(OH)(2)R and [R(OH)SiO-]-(3). Si-29 NMR quantitative analysis was performed on SiP-TEOS mixtures during the hydrolysis-condensation reactions and on the final gels. This provided information on the formation of the gels molecular structure.