Fiber reinforced materials are present in a wide field of industrial applications, due to their good strength-weight-ratio. This is especially true for thermoplastic composites, which have the advantage of high impact tolerances, quick forming processing capability, and weldability. These materials can be joined, for example, by laser transmission welding. For several thermoplastics, the weld seam quality is affected by the moisture content of the joining members. During laser transmission welding, the welding members are heated up and water in the composite evaporates, causing cavities in the weld seam. For reliable joining connections between thermoplastic parts with high moisture content, it is necessary to determine the effect of moisture on the weld seam quality. Therefore, the influence of the moisture content on the weld seam quality was investigated. The joining members consisted of endless glass fiber reinforced polyetherimide and endless carbon fiber reinforced polyetherimide, which were conditioned in three different ways. Afterward, lap shear and cross section samples were prepared and analyzed.