Today, parts based on fiber reinforced thermoplastics are used in many different applications in the aerospace and automotive industry. Furthermore, the number of parts made of semicrystalline polyphenylene sulfide (PPS) or polymers belonging to the polyaryletherketone (PAEK) family are increasing due to their excellent chemical and mechanical resistance. For some applications, these parts have to be joined to more complex structures. Besides mechanical fastening or adhesive bonding, laser transmission welding can be used when one of the joining members consists of a natural or glass fiber reinforced thermoplastic. The transmissivity of the joining members for the wavelength of the laser has an influence on the welding parameters. Often, diode lasers are used emitting in a wavelength range from 808nm to 980nm. At this wavelength range, PPS and PAEK have a lower transmissivity than for example at 1530nm. Therefore, a change of the used wavelength should affect the welding process. In order to determine this influence, a study was conducted comparing the welding process with diode lasers emitting at 940nm and 1530nm focusing on welding times. In these investigations, the joining members were made of glass fiber reinforced PPS and carbon fiber reinforced PPS.