Electrocatalytic activity of a water-soluble nickel complex, [Ni(DHMPE)(2)](2+) (DHMPE = 2-bis(di(hydroxymethyl)phosphino)ethane), for the hydrogen evolution reaction (HER) at pH 1 is reported. The catalyst functions at a rate of similar to 10(3) s(-1) (kobs). with high Faradaic efficiency. Quantification of the complex before and after 18+ hours of electrolysis reveals negligible decomposition under catalytic conditions. Although highly acidic conditions are common in electrolytic cells, this is a rare example of a homogeneous catalyst for HER that functions with high stability at low pH. The stability of the compound and proposed catalytic intermediates enabled detailed mechanistic studies. The thermodynamic parameters governing electron and proton transfer were used to determine the appropriate reductants and acids to access the catalytic cycle in a stepwise fashion, permitting direct spectroscopic identification of intermediates. These studies support 4 mechanism for proton reduction that proceeds through two-electron reduction of the nickel(II) complex, protonation to generate [HNi(DHMPE)(2)](+), and further protonation to initiate hydrogen bond formation.