Samples were taken at various stages of the polycondensation between different silicon alkoxides RSi(OR')3 which are reacted together in the preparation of membranes for ultrafiltration and reverse osmosis. Si-29 NMR spectra could be interpreted by using empirical increments of the chemical shift, which are characterized for substitutions at the Si atom. It is found that the reaction attains the maximum degree of condensation forming a three-dimensionally infinite siloxane RSi(OSi=)3 backbone, which is, however, partially degraded by thermal treatment (120-degrees-C) leading to R2Si(OSi=)2 and Si(OSi=)4 groups. The organic part is not affected during these reactions, and side reactions in the early stages of polymerization (e.g. ligand exchange with the solvent) were reversible.