Poly(dimethylsiloxane) (PDMS) networks have been prepared by tetrafunctionally endlinking vinyl-terminated chains, in a hydrosilylation cure at low temperatures. The results obtained demonstrated the absence of important side reactions, and the resulting networks were studied with regard to their stress-strain isotherms in elongation. Values of the moduli in the large deformation (phantom) limit and small deformation (affine) limit fell within the bounds predicted by the constrained-junction theory. The results do not suggest the presence of significant contributions from trapped entanglements. In this analysis, Factors affecting the determination of the network structural parameters have been emphasized, because of serious experimental difficulties in making perfect networks. it appears that, in the absence thus far of carefully controlled and well-defined stoichiometry in the reacting systems, the procedure best suited for testing the various theories of rubberlike elasticity consists of plotting the sum of the Mooney-Rivlin constants 2C(1) + 2C(2) (representing the low-deformation) modulus against 2C(1) (representing the high-deformation phantom modulus).