The copolymers of acrylic acid(AA) and methacrylic acid(MA) with various compositions were synthesized in benzene using benzoyl peroxide as an initiator. Then the compositions of the copolymers were determined with FT-IR analysis. The reactivity ratios of AA and MA were also determined. The compositions of the copolymers could be determined by FT-IR analysis more easily and precisely than elemental analysis or NMR analysis. The results show that the content of MA unit in the copolymer was greater than the feed ratio of MA in AA/MA copolymerization. The reactivity ratios of AA and MA are 0.668 and 1.530, respectively. Since the product of the two reactivity ratios is approximately unity (rArM 1), this copolymerization can be termed ideal.