The temperature dependence of the interaction parameter X between polystyrene and poly(methyl methacrylate) was studied by means of cloud point and PVT measurements. With these data the phase diagrams and the temperature dependence of chi were calculated using the classical Flory-Huggins theory, the equation-of-state theory of Flory, Orwoll, and Vrij, and a modified cell model by Dee and Walsh. The results obtained from all three theories are very similar, suggesting that the free volume contributions in the system under investigation are of minor influence. The comparison between obtained data and literature data of absolute values of chi is very difficult because a number of model-dependent quantities, such as segment length or segment volume, influence the results. Thus it is always better to compare the temperature dependence of chi. However, also in this case model parameters can lead to different chi(T) functions as shown for the examples of the determination of chi(T) from interfacial thickness data and from phase diagrams using equation-of-state theories.