A series of copolymers of acrylonitrile and vinyl acetate with different composi-tion has been synthesized．Their IR etra have been compared with those of eorres．ponding mixtures of polyaerylonitrile and polyvinyl acetate．Quite distinct differences are found between the IR spectra of copolymers and corresponding mixtures．Such ldnd of differenee is explained in view of the interaction between the adjacent units within a copolymer molecule．Based on the previous observation，the sequence distribution of acrylonitrile units in aerylonitrile-vinyl acetate copolymers has been measured with the shift of nitrile stretching vibration band．The composition relation of copolymers has been measured with the absorbance ratio of carbo*nyl stretching vibration band and nitrile stretching vibration band．The error of these measurements in a quite wide range of copolymer composition is estimated to be less than 3％．