The miscibility of tetramethylpolycarbonate (TMPC) blends with styrenic copolymers containing various methacrylates was examined, and the interaction energies between TMPC and methacrylate were evaluated from the phase-separation temperatures of TMPC/copolymer blends with lattice-fluid theory combined with a binary interaction model. TMPC formed miscible blends with styrenic copolymers containing less than a certain amount of methacrylate, and these miscible blends always exhibited lower critical solution temperature (LOST)-type phase behavior. The phase-separation temperatures of TMPC blends with copolymers such as polystyrene-co-methyl methacrylate), polystyrene-co-ethyl methacrylate), polystyrene-co-n-propyl methacrylate), and polystyrene-co-phenyl methacrylate) increase with methacrylate content, go through a maximum, and decrease, whereas those of TMPC blends with poly(styreneco-n-butyl methacrylate) and polystyrene-co-cyclohexyl methacrylate) always decrease. The calculated interaction energy for a copolymer-TMPC pair is negative and increases with the methacrylate content in the copolymer. This would seem to contradict the prediction of the binary interaction model, that systems with more favorable energetic interactions have higher LCSTs. A detailed inspection of lattice-fluid theory was performed to explain such phase behavior.