Poly(dimethylsiloxane) networks were prepared by tetrafunctionally endlinking hydroxyl-terminated chains with tetraethoxysilane (TEOS). Molecular composites were then prepared by in situ sol-gel reactions on additional TEOS swelled into the networks, resulting in the formation of reinforcing silica fillers within the host elastomers. The amount of filler generated generally increased linearly with an increase in the TEOS swelling ratio, as expected. The silica particles formed were examined by small-angle X-ray scattering. Of particular interest were the relationships between particle size and molecular weight M-c of the network chains (mesh sizes), amount of filler introduced, and catalyst concentration. Particle sizes were smallest for the smallest values of M-c, possibly demonstrating constraining effects from the very short network chains. At fixed M-c and filler concentrations, higher catalyst concentrations gave larger particles. Increase in filler concentration generally had little effect on particle size at low and high loadings, but markedly increased sizes at intermediate levels (10-20 wt %), presumably caused by coalescence of the scattering entities into considerably larger aggregates.