Research into the fusion bonding of joints containing thermoplastic and/or thermoset composites. A thermoplastic interlayer is co-cured with the thermoset that provides a fusible surface. Depending on the type of materials the interlayer can form an adhesive interface or a gradient interphase with the thermoset. Welding of thermoset composite to thermoplastic composite coupons with both interlayer types were performed. High lap shear strength are obtained, even after prolonged exposure to moisture. The joint that uses the gradient interphase has the potential to be certified for aerospace, in contrast with the adhesive interface type which faces the same challenges as seen in the certification of adhesive bonds. This technique provides an alternative to mechanical joining or adhesive bonding of cured thermoset structures.