A simple spectrophotometric method for the evaluation of formation constants for aqueous copper(I) has been developed, based on the kinetics of reduction of Co-III(NH3)(5)X complexes. The method has been applied to the aqueous copper(I)-acetonitrile system to determine the successive formation constants beta (1), beta (2), and beta (3) as 4.3 x 10(2) M-1, 1.0 x 10(4) M-2, and 2.0 x 10(4) M-3, respectively, in 0.14 M NaClO4/HClO4 at 21 +/- 1 degreesC.