A series of novel semi-interpenetrating networks (semi-1PNS) has been prepared and characterized. The semi-TPNs were prepared by blending a commercially available polyetherimide with commercially available acetylene-terminated polyisoimides. Thermal treatment of the polyetherimide/acetylene-terminated polyisoimide blends causes the acetylene-terminated polyisoimide to concurrently rearrange to a polyimide and to crosslink. The resulting semi-IPNs exhibit excellent mechanical performance and chemical resistance. The preparation and characterization of this system are described.