The thermodynamic phase behaviour of blends composed of two random copolymers of polystyrene-co-methyl methacrylate (SMMA) and polystyrene-co-acrylonitrile (SAN) was studied using a modified Florys equation of state (EOS) theory. Light scattering measurements indicated that SMMA and SAN are miscible within certain copolymer composition ranges and exhibit lower critical solution temperature behaviour. The temperature, at which phase separation is observed, changes with the copolymer composition. The segmental interaction energy parameters XS-MMA, XS-AN and XMMA-AN were calculated using a binary interaction model by fitting the lowest temperature in the calculated spinodal boundary from EOS theory to the experimental data for several copolymer compositions. It was found that EOS theory, originally used for binary polymer solutions, could be modified and successfully extended to simulate the phase boundaries of random copolymer blends, when using Flory's mixing rule to calculate the equation of state parameters for the copolymers from those of the constituent monomer segments.