Abstract: This invention is a new process to promote adhesion between thermoplastic polymers and thermosetting resin adhesives. This is accomplished by selecting a third "interlayer" polymer which is compatible with both the thermoplastic and thermoset polymers. This compatible interlayer is incorporated with the thermoplastic polymer during fabrication to provide the finished part surface with a layer of the compatible film. Upon adhesive bonding, diffusion of the thermosetting adhesive molecules into the compatible film occurs before complete cure, or hardening of the thermosetting adhesive. After completion of cure an Interpenetrating Network (IPN) is formed. Molecular entanglement in this network provides superior adhesive strength enhancement.