For some applications of the aerospace and automotive industry, thermoset and thermoplastic composites need to be joined. This material combination cannot be welded because thermoset matrix materials do not melt. In order to apply welding for this material combination, a semi-interpenetrating polymer network (SIPN) layer was generated on the surface of the thermoset material. SIPN-composites consist of a thermoset carrier covered by a thin thermoplastic functional layer. This SIPN-layer can now be used as a connection material for welding. In this work, the authors investigated the process heat development and the weld seam quality of laser transmission welded SIPN-composites to glass fiber fabric reinforced polyetherimide. Furthermore, the influence of an SIPN-layer containing carbon black was compared with that of an unmodified SIPN-layer. For these investigations, bead on plate welding was performed in order to monitor the process heat generation using a thermographic camera. Additionally, lap shear samples consisting of both SIPN-types were welded and tested. These results were correlated with the detected bead on plate welding process temperatures.