Though difficult to prove from experimental data, the concept that network chain density is reflected in the Mooney-Rivlin C1 parameter has received fairly wide acceptance. In the present work, experimental stress strain measurements on natural rubber are shown to be consistent with the idea that the network chain density is directly related to the sum of the Mooney-Rivlin constants, C1 and C2; rather than to the C1 parameter only. The determination of elastically effective chain densities in polydimethylsiloxane networks is also discussed. A relationship is obtained for concentration dependence of the polydimethylsiloxane-toluene interaction parameter. The results are used to study the cure of polydimethylmethylvinylsiloxanes with “vinyl specific” peroxides. Experimental data favor the proposal that a trimethylene crosslink is formed by the reaction of a methyl group with a vinyl group through a free radical mechanism.