Compared to different welding methods such as ultrasonic welding laser transmission welding is a relatively new technology to join thermoplastic parts. The most significant advantages are the contactless energy input which can be controlled very precisely and the low mechanical loads on the welded parts. However there are several problems in welding dissimilar thermoplastics. This may be due to different melting points on the one hand and different polarities on the other hand. So far these problems are faced with the intermediate layer technique. To avoid this additional step, different ways of for joining dissimilar thermoplastics are investigated. As part of a current research project, the improvement in weldability of dissimilar thermoplastics by means of plasma surface modification and contour welding is examined. The upper, transmissive part is made of polyamide 6 (PA 6), while the lower, laser absorbing component is made of carbon black pigmented polypropylene (PP). To evaluate the influence of the plasma surface modification process on the subsequent welding process of the two dissimilar materials, the treatment time as well as the time between treatment and welding are varied. The treatment time is varied from 0.5 s to 120 s and the time between treatment and welding is varied from a few minutes up to a week. As reference, parts made of the same polymer (PP and PA 6) are welded and tested. For the evaluation of the results of the welding experiments short-time tensile tests are used to determine the bond strength. In addition microscopy is performed to assess the morphology of the heat affected zone and contact angle measurements are executed to analyse the influence of the surface modification on the weldability. The results show that the highest bond strengths are achieved when polyamide is treated for a long time and polypropylene for a short period of time. The highest tear-out force of about 1500 N is measured when treating the components that way.