Poly(dimethylsiloxane) elastomers were filled with two zeolites which were both silicates of sodium and aluminum, but which had different cavity sizes. The reinforcement of the particles provided was characterized by stress-strain measurements in elongation at room temperature. The results indicated that the ultimate properties of the networks were increased by both types Of zeolites, but that the increase was larger in the case of the zeolite with the larger cavity size. Small-angle-neutron-scattering results confirmed that the interface between the elastomeric phase and the zeolite particles was smooth, but gave no evidence for increased polymer penetration into the larger cavity zeolite. Transmission electron microscopy (TEM) showed that the zeolite with a larger pore size also had a smaller particle size, and this is probably the origin of its superior reinforcing ability.