The electrochemical reduction of the black dye photosensitizer [(H-3-tctpy)Ru-II(NCS)(3)](-) (H-3-tctpy = 2,2':6',2"-terpyridine-4,4',4"-tricarboxylic acid) used in photovoltaic cells has been found to be a complex process when studied in dimethylformamide. At low temperatures, fast scan rates and at a glassy carbon electrode, the chemically reversible ligand based one-electron reduction process [(H-3-tctpy)Ru(NCS)(3)](-) + e(-) reversible arrow [(H-3-tctpy(...))Ru(NCS)(3)](2-) is detected. This process has a reversible half-wave potential (E-1/2(r)) of - 1585 +/- 20 mV versus Fc/Fc(+) at 25 degreesC. Under other conditions, a deprotonation reaction occurs upon reduction, which produces [(H3-x-tctpy(x-))Ru(NCS)(3)]((1+x)-) and hydrogen gas. Mechanistic pathways giving rise to the final products are discussed. The E-1/2(r)-value for the ligand based reductions of the deprotonated complex is 0.70 V more negative than for [(H-3-tctpy)Ru(NCS)(3)](-). Consequently, data obtained from molecular orbital calculations are consistent with the reaction [(H-3-tctpy)Ru(NCS)(3)] +e(-) --> [(H-2-tctpy(-))Ru(NCS)(3)](2) - + 1/2H(2) yielding the monodeprotonated complex as the major product obtained after electrochemical reduction of [(H-2-tctpy)Ru(NCS)(3)](-). The E-1/2(r)-values for the metal based Ru-II/III process differ by 0.30 V when data obtained for the protonated and deprotonated forms of the black dye are compared. Electronic spectra obtained during the course of experiments in an optically transparent thin layer electrolysis configuration are consistent with the overall reaction scheme proposed on the basis of voltammetric measurements and molecular orbital calculations. Reduction studies on the free ligand, H-3-tcpy, are consistent with results obtained with [(H-3-tctpy)Ru(NCS)(3)](-).