The long controversial nature of the equilibrium phase behaviour in the blend system of poly(methyl methacrylate) (PMMA) with bisphenol A polycarbonate (PC) was investigated by dissolving both polymers in an oligomeric epoxy, namely the diglycidylether of bisphenol A (DGEBA). Phase behaviour and morphology changes with temperature were examined by using differential scanning calorimetry (d.s.c.) and optical microscopy. Upon lowering the T(g)s of the PC/PMMA blends, thus enhancing the chain mobility by the plasticizing epoxy molecules, phase separation did take place in the blend with an accelerated rate at temperatures as low as 68 degrees C. To our knowledge, this has never been reported previously in the literature. Consequently, the widely reported miscibility and lower critical solution temperature (LCST) in PC/PMMA blends have to be carefully reinterpreted. Our results have suggested that the equilibrium behaviour of PC/PMMA blends is actually one of phase separation, with possibly only partial miscibility. In addition, this study has also showed that the single-T-g, transparent blends above 240 degrees C of the reported ’upper critical solution temperature (UCST)’ are actually not miscible but display microphase domains. A reinterpretation was provided based on these results.