This study examines the phase behavior of ethylene-x,inyl acetate copolymer (EVA) and poly(methyl methacrylate) (PMMA) blends during MMA polymerization. The ternary PMMA/MMA/EVA mixtures are considered to create a triangular phase diagram, which responds the phase changes during polymerization. The phase changes during MMA polymerization are also examined by optical microscope and photometer. Since the PMMA and EVA are well-known immiscibles, the polymer solution undergoes phase separation at the initial stage of the MMA polymerization. Additionally, the phase inversion occurs as the conversion of MMA between 13.8 and 20.8%. On the other hand, the EVA-graft-PMMA, which can reduce the dispersed EVA particle size, is induced efficiently by taking tert-butyl peroctoate (t-BO) as initiator during MMA polymerization. (C) Wiley Periodicals, Inc.