An indirect and alternative method for the determination of sulfate ion has been established by spectrophotometry. The procedure is based on the formation of copper(II) monoazido species, in water/acetone medium, in which a discoloration effect of system is caused by sulfate presence. In the recommended conditions, sulfate ions can be determined in the concentration range from 40 to 5000 mg/l (ppm), at 375 nm, in natural samples of waters. The best experimental conditions were determined analysing the different factors involved. Studies about the formation equilibria of the CuN3+and CuSO4species were realized in water and their respective stability constants (β1= 225 M−1and 7.15 M−1) were determined by spectrophotometry at 25.0° C and ionic strength 1.0 M (NaClO4). Equilibrium investigations were also made in presence of acetone. Additional studies on the ternary system Cu(II)/N3−/SO42-) indicated some evidences of mixed complexes.