The condensation of iso-butyltrimethoxysilane and gamma-methacryloxypropyltrimethoxysilane (gamma-MPS) in aqueous solutions has been studied. The silanes were hydrolysed in acidic solutions followed by adjustment of the pH of the solutions to allow condensation at different pH conditions. The molecular weights of the polymer mixtures obtained were determined by using gel permeation chromatography (GPC) with a light scattering detector. The GPC results suggest that the polybutylsilsesquioxanes differ in molecular weight distribution to the polymethacryloxypropylsilsesquioxanes. In general, polymers with the highest molecular weight were obtained in mild alkaline conditions. Strongly acidic as well as strongly alkaline conditions gave low molecular weight oligomers. The effect of fluoride ions on polymer molecular weight was also investigated. The polymers from the hydrolysis and condensation of iso-butylsilane and gamma-MPS were monodisperse when the condensation was catalysed by fluoride ion. The molecular weights of the polymers were lower than those from the analogous base-catalysed reactions and the molecular weights increased with an increase in fluoride concentration.