The controversial equilibrium phase behaviour in the blend system of poly(methyl methacrylate) and bisphenol-A polycarbonate was re-examined by using differential scanning calorimetry, optical and scanning electron microscopy. This study has shown that the ’transparent’ blends of PC/PMMA actually displayed micro-heterogeneity, with domain sizes of ca. 100-300 Angstrom. The micro-heterogeneous domains were so small that the blends exhibited a single glass transition point (T-g) and appeared transparent. Upon heating above the T-g of the matrix components of the blends, the original micro-phase domains suddenly expanded at 180 degrees C, increasing rapidly to about 1-3 mu m in size, with increased cloudiness. This present study has also shown that the ’transparent’ blends when heated above 240 degrees C also displayed micro-phase separation. Consequently, the phase behaviour in the PC/PMMA blends are re-interpreted from kinetic points of view.