Hydrolysis of tetraalkoxysilanes leads to the formation of organosiloxane polymers by polycondensation reactions. This process is strongly affected by certain parameters: molecular separation of species, their ratios, the reaction medium, the concentration of catalyst, the reaction length, and the temperature. The general effects of these parameters on the polymeric make-up and molecular size distribution during the hydrolytic polycondensation of Si(OC₂H₅)₄ were investigated by using high resolution nuclear magnetic resonance (NMR), and size-exclusion liquid chromatography (SEC). The results show that each of these parameters, particularly the molecular separation during the hydrolytic polycondensation affect the molecular structure in a fundamental way. There is also evidence that the molecular size expansion occurs by two distinct processes: initially by a gradual growth process, and later by recombination of high-molecular weight species. The latter process often leads to a bi-modal molecular size distribution. -- AATA