The miscibility and lower critical solution behaviour of pol(ethyl methacrylate) with poly(styrene-stat-acrylonitrile) have been studied. Poly(ethyl methacrylate) is miscible with styrene/acrylonitrile copolymers having acrylonitrile contents between 9 and 34 wt %. By increasing the size of the pendant group from methyl to ethyl, the repulsive interaction between the methacrylate and acrylonitrile increases, but that between the methacrylate and styrene decreases.