The miscibility of poly(methyl methacrylate) (PMMA) and styrene-acrylonitrile random copolymers (SAN) blends was investigated on the basis of the Flory-Orwoll-Vrij equation of state theory. To obtain the equation of state parameters (P*, V*(sp), T* : characteristic parameters), the pressure-volume-temperature (PVT) behaviour was measured for PMMA and a series of SANs with various acrylonitrile contents. The exchange energy parameter X(ij) was also calculated by fitting the theory to some phase diagrams of PMMA/SAN blends. The Flory-Huggins interaction parameter X : was separated into two contributions based on the equation of state theory for mixtures : the exchange energy term chi(inter) and the free volume term chi(free). Both the temperature and copolymer composition dependences of chi(inter) and chi(free) were estimated by calculations using the equation of state parameters. There exists a region in which X(inter) is negative, leading to a miscibility window in PMMA/SAN blends. However, the immiscibility at high temperatures in the blends cannot be explained only by X(inter); it is caused by the free volume contribution, chi(free). The miscibility window behaviour in PMMA/SAN blends may be explained within the framework of the equation of state theory.