Today, fiber reinforced materials are present in many industrial applications to implement lightweight solutions. These materials are used for example in automotive, aerospace and medical sectors. In the last couple of years, fiber reinforced materials with a thermoplastic matrix have gained importance due to their thermo formability and good recyclability. Besides the joining of these materials by mechanical fasteners, they can also be joined by welding in order to generate complex parts. A new welding method for composites is laser transmission welding. This welding technique has high potential for excellent reproducibility, high flexibility, and automation. The weld seam quality of several kinds of thermoplastics can be affected by the moisture content of the matrix material. During laser transmission welding, both parts are heated up at the interface. Due to the process heat, moisture in the composite evaporates and can cause pores in the connection area. The authors investigated the influence of the moisture content on the weld seam quality of endless glass fiber reinforced polyetherimide (GF PEI) welded to GF PEI containing carbon black. Before welding, the material was conditioned in three different ways to generate “dry”, “wet” and “room humid” samples. With the materials, lap shear samples were produced with different welding parameters. The lap shear strength results were correlated to the moisture content and the utilized welding parameters. Furthermore, cross sections were prepared to determine the amount of pores in the joining zone and correlated to the lap shear strength results.