The phase behaviour of binary blends of styrene-methacrylonitrile (SMAN) copolymers and methyl methacrylate copolymers with cyclohexyl methacrylate and t-butyl methacrylate has been determined as a function of composition of each copolymer at 130, 150 and 180-degrees-C. These miscibility maps are compared with those of recently reported blends of styrene-acrylonitrile (SAN) copolymers with methacrylate copolymers. Blends of SMAN with methacrylate copolymers have a much wider miscibility area than the corresponding blends of SAN with methacrylate copolymers. From these miscibility maps, binary interaction energy densities, B(ij), have been calculated for various monomer unit pairs at each temperature. The approximate temperature dependence of B(ij) for each monomer unit pair was also determined. The interaction of methacrylonitrile with styrene, cyclohexyl methacrylate and t-butyl methacrylate is repulsive (positive) but much less repulsive than that of acrylonitrile with the other monomer units. The results show that the alpha methyl group in the methacrylonitrile unit greatly affects the interaction energy density with other monomer units. Similar effects of the alpha methyl group have been reported recently by examining the miscibility of blends of poly(methyl methacrylate) with poly(methyl acrylate) or poly(ethyl acrylate). The effects of these issues on phase behaviour are discussed.