Abstract: A sensitive spectrophotometric method for arsenic analysis is described. The higher sensitivity comes from the effect of ethyl alcohol on the reduction reaction of antimon-heteropolymolybdic acid. The sensitivity of this method is 50 times higher than the normal arsenoantimonomolybdate method. The detection limit(based on three times the standard deviation of ten blank measurment) is 18ppt (50ml water sample) and the coefficient of variation is 0.8% and 3.8% at the blank solution and the 0. 3 μg level, respectively.The effect of ethyl alcohol is discussed preliminarily. The reduction reaction of antimon-heteropolymolybdic acid was catalyzed by As(Ⅴ) only when ethyl alcohol or other organic solvents (in water) such as methyl alcohol, isopropanal, dimethylformamide, dioxane and ice acetic acid exists in the arsenoheteropolymo-lybdate solution.