In recent years, laser transmission welding has gained in significance by displaying its specific advantages among the established welding processes for thermoplastics. However, a deep understanding of the developed process variants is so far missing. Useful results for temperature development were obtained in cases of high absorption constants by setting up an analytical model by analogy to single-sided heat impulse welding. Yet there is no physico-mathematical model considering the different energy conditions for joining parts with various absorption properties. This investigation is a first step towards a deep and detailed insight into the heating phase of the laser transmission welding process. Experimental data for temperature progression was collected for polypropylene. In addition, an analysis of the heat transfer problem using the finite element method showed a good level of agreement with the experimental results.