A series of cobaloxime complexes-([Co(dmgH)(2)pyCl] (1), [Co(dmgH)(2)(4-COOMe-py)Cl] (2), [Co(dmgH)(2)(4-Me2N-py)Cl] (3), [Co(dmgH)(dmgH(2))Cl-2] (4), [Co(dmgH)(2)(py)(2)](PF6) (5), [Co(dmgH)(2)(P(n-Bu)(3))Cl] (6), and [Co (dmgBF(2))(2)(OH2)(2)] (7), where dmgH = dimethylglyoximate monoanion, dmgH(2) = dimethylglyoxime, dmgBF2 = (difluoroboryl)dimethylglyoximate anion, and py = pyridine-were synthesized and studied as molecular catalysts for the photogeneration of hydrogen from systems containing a Pt terpyridyl acetylide chromophore and triethanolamine (TEOA) as a sacrificial donor in aqueous acetonitrile. All cobaloxime complexes 1-7 are able to quench the luminescence of the Pt(II) chromophore [Pt(ttpy)(C CPh)]ClO4 (C1) (ttpy = 4'-p-tolyterpyridine). The most effective electron acceptor for hydrogen evolution is found to be complex 2, which provides the fastest luminescence quenching rate constant for C1 of 1.7 x 10(9) M-1 s(-1). The rate of hydrogen evolution depends on many factors, including the stability of the catalysts, the driving force for proton reduction, the relative and absolute concentrations of system components (TEOA, Co molecular catalyst, and sensitizer), and the ratio of MeCN/water in the reaction medium. For example, when the concentration of TEOA increases, the rate of H-2 photogeneration is faster and the induction period is shorter. Colloidal cobalt experiments and mercury tests were run to verify that the system is homogeneous and that catalysis does not occur from in situ generated colloidal particles during photolysis. The most effective system examined to date consists of the chromophore C1 (1.1 x 10(-5) M), TEOA (0.27 M), and catalyst complex 1 (2.0 x 10(-4) M) in a MeCN/water mixture (24:1 v/v, total 25 mL); this system has produced similar to 2150 turnovers of H-2 after only 10 h of photolysis with lambda > 410 nm.