Thermoplastic composite structures based on continuous carbon fibre reinforcements are gaining in importance in many industrial applications. These comprise basic technical functions as well as high performance applications in the aerospace sector. In contrast to thermoset systems, for composites based on thermoplastic matrix materials, e.g. polyamide, polyphenylene sulfide or polyetherether ketone, welding techniques are applicable. In this context, welding steps are not limited to the joining of CFRP parts among themselves, but extend to the connection of diverse components, consisting of unreinforced and glass fibre reinforced thermoplastics, to CFRP components. In this work, a laser transmission welding process is evaluated with respect to the influence of the carbon fibre reinforcement within the laser absorbing part as well as the glass fibre reinforcement within the laser transparent part on the weld seam formation. As thermoplastic base materials, nylon (PA66) and polyphenylene sulfide (PPS) are used. By applying different strategies in terms of contour and quasi-simultaneous welding, the influence of this group of endless fibre reinforced composites on the welding process is studied. Significant differences to the process characteristics known from the joining of unreinforced thermoplastics, emerge from the fibre reinforcement inducing high thermal conductivity and fluctuating absorption properties for the diode laser wavelength, resulting in an essentially altered plastification performance which is directly mirrored in the inhomogeneous formation of the weld seam structure. Corresponding temperature distributions are monitored by use of pyrometric sensors