Abstract: Binary polymer styrene-acrylonitrile was prepared by polymerization of styrene and acrylonitrile free radicals induced by azobisisobutyronnitrile (AIBN).A series of binary polymers with different fractions of acrylonitrile was prepared.Two infrared absorption peaks at wave numbers of 705 cm-1 and 2 240 cm-1 were selected for the determination of styrene and acrylonitrile in binary polymers,respectively.Linearity relationship between the ratio of styrene absorbance to the acrylonitrile absorbance with the reciprocal molar fraction of acrylonitrile in the binary polymer was presented.Working curve was established on the basis of regression analysis of the above mentioned data.