Abstract While chemically different monomers can be mixed, the mixing of the corresponding polymers in the solid state usually results in (multiphase) polyblends. Even the mixing of two different polymer solutions generally produces two liquid phases. This difficulty can be overcome, without resorting to grafting, by producing interpenetrating polymer networks (IPN). By crosslinking an initially linear polymer in the presence of a crosslinked network we can produce a polymeric catenane or an IPN held together predominantly by “permanent” entanglements, assuming chain transfer reactions can be suppressed. Several workers have made IPN's from either chemically identical or chemically similar polymers and have studied their properties. In this work, two different elastomers, a crosslinked polyether-based poly(urethane-urea) and a linear poly(butadiene-acrylonitrile), as aqueous emulsions, are mixed, together with crosslinking agents and stabilizers, films cast, and subsequently cured to form the IPN. A partial chemical structure proof is obtained by separating the crosslinked poly(butadiene-acrylonitrile) from the IPN by essentially total hydrolysis of the interpenetrating poly(urethane-urea) network.