Elastomeric molecular networks have been prepared by endlinking polydimethyl siloxane molecules having functional chain ends, both in the presence of an unreactive polymeric diluent and in the undiluted state. Values of tensile (Young) modulus were found to be in good agreement with the simple molecular theory of rubberlike elasticity for networks prepared in a highly diluted state. For concentrated systems the modulus was anomalously high, however. The discrepancy can be attributed to chain entanglements. A second interpenetrating network was introduced into networks formed in the diluted state by replacing the diluent polymer by reactive polymer, which was then gelled in situ. The modulus of these combined networks was much higher than the sum of the moduli of the constituent networks, implying a large contribution from molecular entanglements.