Methacrylates have high glass transition temperature (T-g) values and high thermal stability. Anew methacrylate copolymer, poly(4acetylphenyl methacrylate-co-ethyl methacrylate) (APMA-co-EMA), was synthesized. The thermal behaviors of copolymers were investigated by differential scanning calorimetry and thermogravimetric analysis. They behaved as new single polymers with single T-g ' s and the thermal stability of the copolymers increased with increasing 4-acetylphenyl methacrylate (APMA) fraction, leading to the manufacture of copolymers with desired T-g values. Structure and composition of copolymers for a wide range of monomer feed ratios were determined by Fourier transform infrared (FT-IR) and 1H-nuclear magnetic resonance (1H-NMR) spectroscopic techniques. Copolymerization reactions were continued up to 40% conversions. The monomer reactivity ratios for copolymer system were determined by the Kelen-Tudos (r(a(APMA)) = 0.81; r(a(EMA)) = 0.61) and extended Kelen-Tudos (r(a) = 0.77;r(b) = 0.54) methods and a nonlinear least squares (r(a) = 0.74;r(b) = 0.49) method.