The nitrite reductase (Nir) isolated from Pseudomonas chlororaphis DSM 50135 is a blue enzyme, with type 1 and type 2 copper centers, as in all copper-containing Nirs described so far. For the first time, a direct determination of the reduction potentials of both copper centers in a Cu-Nir was performed: type 2 copper (T2Cu), 172 mV and type 1 copper (T1Cu), 298 mV at pH 7.6. Although the obtained values seem to be inconsistent with the established electron-transfer mechanism, EPR data indicate that the binding of nitrite to the T2Cu center increases its potential, favoring the electron-transfer process. Analysis of the EPR spectrum of the turnover form of the enzyme also suggests that the electron-transfer process between T1Cu and T2Cu is the fastest of the three redox processes involved in the catalysis: (a) reduction of T1Cu; (b) oxidation of T1Cu by T2Cu; and (c) reoxidation of T2Cu by NO(2) (-). Electrochemical experiments show that azurin from the same organism can donate electrons to this enzyme.