Remarkable accelerating effects of the ammonium cation (NH 4 +) have been observed on photoinduced electron-transfer reactions from the triplet excited state of tetraphenylporphyin (H 2P) to quinones [p-benzoquinone (Q) and naphthoquinone (NQ)] in dimethyl sulfoxide (DMSO). The tetrabutylammonium cation (NBu 4 +) is also effective to accelerate the electron-transfer reduction of Q and NQ in dichloromethane (CH 2Cl 2). The hydrogen bonding interaction between the semiquinone radical anion (Q •- or NQ •-) and NH 4 + was confirmed by the ESR spectra of the Q •-/NH 4 + and Q •-/(NH 4 +) 2 [NQ •/NH 4 + and NQ •/(NH 4 +) 2] complexes in DMSO. Accelerating effects of NH 4 + in DMSO and NBu 4 + in CH 2Cl 2 on the rates of photoinduced electron-transfer reduction of Q by H 2P result from the positive shift of the E red value of quinones together with a constant E ox value of H 2P determined from cyclic voltammetry measurements. The driving force dependence of the rate constants for photoinduced electron-transfer reduction of quinones in the presence of various concentrations of ammonium cations was evaluated in light of the Marcus theory of electron transfer.