A technique was developed for preparing poly(dimethylsiloxane) nanocomposites having unusually high transparencies as quantitatively judged by ultraviolet-visible spectroscopy. The method was the in situ generation of silica particles by a two-step sol-gel procedure in which the required water of hydrolysis was simply absorbed from the air, and the catalyst was generated in situ from a tin salt. Electron microscopy showed that the phase-separated silica domains were very small (30-50 nm in diameter) and well dispersed, as expected from the transparency of the composites. Stress-strain measurements in tension indicated that the particles provide very good reinforcement. Ultra-small-angle X-ray scattering data showed that the domain morphology depends strongly on catalyst, but weakly on loading level. (C) 2003 Wiley Periodicals, Inc.