A PDMS network, synthesized from a vinyl-terminated precursor, was reinforced by plate-like montmorillonite (volclay) particles with different surface cations. The optimal ratio of crosslinker-to-PDMS precursor was ascertained from the mechanical properties of networks prepared with different crosslinker concentrations. The elastic modulus of the polymer was enhanced by the montmorillonite particles. The increase in modulus was higher in the Li- than in the Na-volclay composites. The ultimate strength of the composites was also strongly enhanced by the small platelets, especially in presence of surface Li+. The stronger influence of Li-volclay on the mechanical properties of the composites can be attributed to the partial formation of an intercalated structure, which leads to thinner particles with a high aspect ratio. Both composite strength and modulus were proportional to the filler-volume-fraction, but the increase in strength was limited by rising particle agglomeration at high loading. In contrast to organic-modified montmorillonite, the inorganic surface of volclay catalyzed the thermal degradation of PDMS.