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 networks (SIPN) layer was generated on the surface of 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 to an unmodified SIPN-layer. For these investigations bead on plate welding was performed in order to monitor the process heat generation by a thermographic camera. Additionally, lap shear samples were welded and tested consisting of both SIPN-types. These results were correlated to the detected bead on plate welding process temperatures.