A detailed mechanistic analysis is presented for the hydrogen evolution catalyst [Ni((P2N2Ph)-N-Ph)(2)(CH2CN)][BF4](2) in acetonitrile ((P2N2Ph)-N-Ph = 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane). This complex has a Ni-II/I redox couple at -0.83 V and a Ni-I/0 redox couple at -1.03 V versus Fe+/0. These two closely spaced redox events both promote proton reduction catalysis, each via a distinct mechanism: an electrochemical ECEC pathway and an EECC route. The EECC mechanism, operative at more negative potentials, was isolated through use of a weak acid (anilinium, pK(a) = 10.6 in CH3CN) to avert protonation of the singly reduced species. Electroanalytical methods and time-resolved spectroscopy were used to analyze the kinetics of the elementary steps of hydrogen evolution catalysis. The rate constant for the formation of a nickel(II)-hydride intermediate was determined via measurements of peak shift (k(1) = 1.2 X 10(6) s(-1)) and through foot-of-the-wave analysis (k(1) = 6.5 x 10(6) M-1 s(-1)). Reactivity of the isolated hydride with acid to release hydrogen and regenerate the nickel(II) complex was monitored by stopped-flow spectroscopy. Kinetics obtained from stopped-flow measurements are corroborated by current plateau analysis of the catalytic cyclic voltammograms. These kinetic data suggest the presence of an off-cycle intermediate in the reaction.