We have measured the equilibrium swelling of polystyrene networks by linear deuterated polystyrene using the ion beam technique of helium-3 nuclear reaction analysis. These measurements enable an analysis of the swelling of networks at low degrees of swelling, and our results showed more swelling than predicted by using the usual forms of Flory−Rehner theory. The swelling of the networks by linear polymers can be explained if we consider the network as connected heterogeneous clusters which unfold during the swelling process. Reasonable results can also be obtained in terms of the Flory−Rehner model if a value of the Flory−Huggins interaction parameter is used that is dependent on the density of cross-links in the network. However, the resultant interaction parameter is negative, in contrast with the general result for isotopic mixtures. We were also able to swell the networks with linear polystyrene, which was initially mixed with high molecular weight polystyrene (which could not penetrate the network),...