Although poly(ethyl methacrylate) (PEMA) and poly(methyl methacrylate) (PMMA) are only slightly different in structure, they are known to be immiscible. Polystyrene is not miscible with PEMA or PMMA. However, when polystyrene is modified to contain certain vinyl phenol groups to become poly(styrene-co-vinyl phenol) (PSVPh), it can be miscible with both PEMA and PMMA. What is the miscibility of a ternary blend consisting of PEMA, PMMA, and PSVPh? For this question to be answered, binary blends of PEMA (or PMMA) were first made with PSVPh. Their miscibility was examined. Then, ternary blends composed of PEMA, PMMA, and PSVPh were prepared and measured calorimetrically. The role of PSVPh between PEMA and PMMA and the effect of different contents of vinyl phenol groups on the miscibility of the ternary blends were investigated. On the basis of experimental results, increasing the vinyl phenol contents of PSVPh seemed to have an adverse effect on the miscibility of the ternary blends. (C) 2003 Wiley Periodicals, Inc.