This study focuses on the preparation of compatible blends with the poly(methyl methacrylate) (PMMA) using a reactive terpolymer maleic anhydride-styrene-vinyl acetate (MA-St-VA). In the first series of experiments, binary blends of the PMMA and the MA-St-VA terpolymer have been prepared in tetrahydrofurane. The PMMA and the MA-St-VA terpolymer formed the compatible blends. The effects on thermomechanical properties of MA-St-VA terpolymer ratio in the blends were studied. The glass transition temperatures (T-g), thermal expansion coefficient (alpha), and other thermomechanical parameters for the blends have been established by TMA method and the compatibility of two polymers has been evaluated by these TMA parameters. The addition of MA-St-VA terpolymer to PMMA made a plasticizing effect on PMMA. This effect regularly changed with the increasing of the terpolymer in the blends. (c) 2006 Wiley Periodicals, Inc.