The miscibility and phase behavior in blends of poly(vinyl butyral) (PVB) and atactic poly(methyl methacrylate) (PMMA) were studied by dynamical mechanical thermal analysis (DMTA) and optical microscopy (OM). PVB is regarded as a random copolymer containing vinyl butyral (VB) and vinyl alcohol (VA) units. In this study the copolymer Composition was varied over a wide range; and PMMA was varied with respect to molecular weight. DMTA measurements show two distinct T-g's for the blends of all PVBs with PMMAs of high molecular weights (M-W = 1.2 x 10(5) and 1.5 x 10(4)), indicating completely immiscibility. However, OM observations show that the size of phase-separated domains of PVB blends with PMMA (M-W = 1.2 x 10(5)) depends on copolymer composition and displays a minimum in the range 22-53 wt % VA content. For low molecular weight PMMAs (M-W = 2 x 10(3) to 5 x 10(3)), miscibility with UCST-type phase behavior was observed in certain blends, the miscibility range depending on the copolymer composition and the molecular weight of the PMMA. The segmental interaction parameters of chi (VA), (MMA), chi (VB), MMA and chi (VA),(VB) and their temperature dependence were determined from the study. The copolymer composition dependence of the averaged interaction parameter between PVB and PMMA, chi (12), was then calculated and displayed a minimum around 30 wt % VA content. These finding can all be interpreted in terms of a copolymer effect in which the unfavorable interaction between PVB and PMMA is minimized at a certain copolymer compositions due to the repulsive interaction between VA and VB units within the PVB chains.