The binary interaction energies between styrene and various methacrylates were determined from newly examined phase boundaries with lattice-fluid theory. Because the blends of polystyrene (PS) and poly(cyclohexylmethacrylate) (PCHMA) were only miscible at high molecular weights when the blends were prepared by solution casting from tetrahydrofuran, we examined the miscibility of other blends by changing the molecular weights of PS or methacrylate polymers. On the basis of the phase-separation temperature caused by the lower critical solution temperature, the miscibility of PS with the various methacrylates appeared to be in the order PCHMA Ͼ poly(n-propyl-methacrylate) (PnPMA) Ͼ poly(ethyl methacrylate) (PEMA) Ͼ poly(nbutyl-methacrylate) (PnBMA) Ͼ poly(iso-butyl-methacrylate) Ͼ poly(methyl methacrylate) (PMMA) Ͼ poly(tert-butyl methacrylate), and the branching of butylmethacrylate appeared to decrease the miscibility with PS. The interaction energies between PS with various methacrylates obtained from phase boundaries with lattice-fluid theory reached minimum value corresponding to the styrene/n-propylmethacrylate interaction. They were in the order PnPMA Ͻ PEMA Ͻ PCHMA Ͻ PnBMA Ͻ PMMA. The difference in the order of miscibility and interaction energies might be attributed to the terms related to the compressibility. The phase-separation temperatures calculated with the interaction energies obtained here indicated that the PS/PEMA and PS/ PnPMA blends at high molecular weights were miscible, whereas the PS/PnBMA blends were immiscible at high molecular weights.