A [C,N] cyclometalated Ir complex, [Ir III(Cp*)(4-(1H- pyrazol-1-yl-κN 2)benzoic acid-κC 3)(H 2O)] 2SO 4  2·SO 4, was reduced by aliphatic alcohols to produce the corresponding hydride complex [Ir III(Cp*)(4-(1H-pyrazol-1-yl-κN 2)-benzoate- κC 3)H] -4 at room temperature in a basic aqueous solution (pH 13.6). Formation of the hydride complex 4 was confirmed by 1H and 13C NMR, ESI MS, and UV-vis spectra. The [C,N] cyclometalated Ir-hydride complex 4 reacts with proton to generate a stoichiometric amount of hydrogen when the pH was decreased to pH 0.8 by the addition of diluted sulfuric acid. Photoirradiation (λ > 330 nm) of an aqueous solution of the [C,N] cyclometalated Ir-hydride complex 4 resulted in the quantitative conversion to a unique [C,C] cyclometalated Ir-hydride complex 5 with no byproduct. The complex 5 catalyzed hydrogen evolution from ethanol in a basic aqueous solution (pH 11.9) under ambient conditions. The 1,4-selective catalytic hydrogenation of β-nicotinamide adenine dinucleotide (NAD +) by ethanol was also made possible by the complex 1 to produce 1,4-dihydro-β-nicotinamide adenine dinucleotide (1,4-NADH) at room temperature. The overall catalytic mechanism of hydrogenation of NAD +, accompanied by the oxidation of ethanol, was revealed on the basis of the kinetic analysis and detection of the reaction intermediates.