The synthesis of methyl methacrylate−phenyl maleimide (MMA−PMI) and methyl methacrylate−tribromophenyl maleimide (MMA−TBPMI) copolymers and the phase behavior of their blends are reported. Copolymer compositions that form miscible binary blends with poly(methyl methacrylate) (PMMA) and polycarbonate (PC) and with copolymers of styrene−acrylonitrile (SAN) and styrene−maleic anhydride (SMA) were established. Repeat unit interaction energies were estimated from isothermal phase boundaries using a binary interaction model and the Flory−Huggins theory. All binary interactions with PMI and TBPMI were found to be endothermic. In general, the interactions with TBPMI are more favorable than with PMI, notwithstanding the interaction with AN where the opposite is true. LCST phase separation was measured for selected blends and compared with predictions based on the Sanchez−Lacombe equation of state.