Modeling laser transmission welding of thermoplastics requires knowledge of the optical properties of the materials being joined. The optical properties are highly dependent on their unique combinations of base polymer, pigments, and fillers. Because of the complex phase transition that occurs when heating thermoplastics, the optical properties are not a constant, but a function of temperature. During laser transmission welding large changes in the material temperature affect the light absorption, thus changing the characteristics of heating owing to the laser radiation. This paper discusses an experiment measuring diode laser transmission through clear poly(vinyl chloride) samples while increasing the material temperature in an oven. It was found that the absorption coefficient follows repeated peaks and valleys between the glass transition temperature and the melting point. Above the melting point the absorption coefficient increases to a plateau before a further increase that is marked by thermal decomposition. In addition, the decomposition temperature was found to be dependent on the rate of heating.