Laser transmission welding (LTW) causes a temperature rise at the weld interface which leads to melting, molecular diffusion and ultimately joining of the two components. Weld temperatures increase with laser power at a given scan speed. However, at higher temperatures, it has been observed that weld strength starts to decline due to material thermal degradation. Thermal degradation is a kinetic phenomenon which depends on both temperature and time. Thermal gravimetric analysis (TGA) is used to study the thermal degradation of two commonly used thermoplastic materials: polycarbonate (PC) and polyamide 6 (PA6). Each material was studied at several levels of carbon black (CB). The TGA data were then used to obtain the kinetic triplets (frequency factor, activation energy and reaction model) of the materials using a non-linear model-fitting method. These kinetic triplets were combined with temperature-time data obtained from a finite element method (FEM) simulation of the LTW process to predict material degradation. The conditions predicted to cause thermal degradation were then compared with experimental data. It is found that the predicted onset of material degradation is in reasonable agreement with both the onset of experimentally observed degradation and the onset of weld strength decline for PC and PA6.