Transparent blends were obtained by melt mixing with a copolymer of methyl methacrylate, 2,4,6-tribromophenyl methacrylate, and cyclohexyl methacrylate (MBC1) and polycarbonate (PC). Differential scanning calorimetry and dynamic mechanical measurements suggested that the blends are thermodynamically miscible. To investigate the miscibility quantitatively, we measured the cloud point curve, from which LCST phase behaviour of the MBC1/PC blend was confirmed below the thermal degradation temperature. Phase demixing process of the MBC1/PC (20/80) blend was measured by light scattering to estimate the spinodal temperature. By analysis with the linearized theory on spinodal decomposition and the mean field theory on free energy, the spinodal temperature was elevated to be 266.4 degrees C. This result was in good agreement with that of the cloud point measurement. From the point of view of polymer-polymer miscibility, the contribution of comonomer unit in MBC1 was discussed.