Difunctional hydroxyl-terminated, and vinyl-terminated poly(dimethylsiloxane) (PDMS) precursors with similar molecular weight distributions were used to synthesize end-linked networks in bulk with appropriate tetrafunctional cross-linkers. Composite PDMS elastomers from the same precursors were also synthesized with low concentrations of montmorillonite nanosize clay particles. For unfilled networks, larger amounts of tetraethyl orthosilicate (TEOS) cross-linkers than conventionally used led to optimal networks with higher moduli and lower soluble fractions. In the montmorillonite-PDMS elastomeric composites, enhancement of the modulus was obtained only for nonoptimal networks formed with the hydroxyl-terminated precursor chains but not with the vinyl-terminated chains. These results indicate that the reinforcement in these elastomers can be attributed to the anchoring of the hydroxyl end group to the silicate filler that dramatically reduces the soluble fraction and binds pendent chain ends. The modulus of the optimal networks could not be enhanced by clay reinforcement.