Sequential interpenetrating polymer networks (IPNs) were prepared by free-radical polymerization. One of the components of the lPN was a poly(butyl acrylate) (PBA) network, and the other one was a poly(methyl methacrylate-co-hydroxyethyl methacrylate) copolymer network. Dynamic-mechanical experiments show that the IPNs are phase separated: two main a relaxations occur in all samples, the low temperature one corresponding to the PBA network and that appearing at higher temperature due to the copolymer network. The latter shows a shape analogous to a pure poly(hydroxyethyl methacrylate) (PHEMA) network independently of the copolymer composition. The influence of water absorption on the dynamic-mechanical spectrum shows that only a small amount of water reaches the butyl acrylate segments. The dependence of the mechanical behavior of the poly(methyl methacrylate-co-hydroxyethyl methacrylate) copolymer networks with the copolymer composition has been also analyzed.