AbstractSolvent‐cast films of polycarbonate (PC)/polymethyl methacrylate (PMMA) mixtures exhibit a single‐phase structure with outstanding optical clarity. Differential scanning calorimetry (DSC) studies show a single glass transition in the intermediate blends. However, the blends undergo phase separation upon heating above 240°C. The phase diagram has been established subsequently by means of a cloud point method and is reminiscent of lower critical solution temperature (LCST) in chara