The coefficient of the decrease of the interfacial tension of polypropylene/polystyrene with temperature is considerably higher than the value of other polymer pairs. This coefficient can be estimated by considering the change of the interaction parameters with temperature, but this approach fails for polypropylene/polystyrene, and other mechanisms are expected to play a role. Ln this paper it is shown that polypropylene starts to degrade at higher temperatures, leading to smaller polymer chains, which decrease the interfacial tension. Besides the change of the interaction parameters with temperature, these smaller molecules also contribute to the temperature coefficient, leading to an apparently high coefficient, The smaller molecules, however, lead to a permanent lower interfacial tension, e.g. the interfacial tension of polypropylene/polystyrene at 200 degrees C is 4.9, 3.9, and 3.0 mN/m, if the polypropylene is first processed at 200, 250, and 300 degrees C, respectively.