Cu-2 (+) is reduced in the presence of nitroprusside to form two Cu(I) reduced nitroprusside species at about + 0.050 V (pH 7.6). These species are reduced further at about - 0.60 V. The two species are formed by an EC mechanism, and the species are believed to be [(CuFe)-Fe-I(CN)(4)NO](-), which predominates in acidic solution, and [(CuFe)-Fe-I(CN)(5)NO](2-), which predominates in alkaline solution. These conclusions are supported by cyclic voltammetric and bulk electrolysis/coulometric experiments.