Polysiloxane-thermoplastic interpenetrating polymer networks (IPNs) are a unique class of polymer-polymer composites that significantly extend the process and mechanical property range of silicone elastomers and allow modification of the behavior of thermoplastics. The materials are characterized by reactive processing consistent with traditional thermoplastic technology and the development of physical properties consistent with semi-IPNs generated from polysiloxanes cross-linked in a variety of thermoplastic matrices, including polyurethanes, polyamides, and polyolefins. The semi-IPNs are prepared by melt-mixing hydride- and vinyl-functionalized polysiloxanes with thermoplastics and inducing cross-linking with a platinum catalyst in a second melt-process step. Polysiloxane composition, cross-link density, and total content affect the process and mechanical behavior of the semi-IPNs. Polysiloxane-thermoplastic IPNs may be used in biomedical materials, gears and bearings, and applications in which the dimensional stability of crystalline resins must be enhanced.