Remarkable accelerating effects of the ammonium cation (NH4+) have been observed on photoinduced electron-transfer reactions from the triplet excited state of tetraphenylporphyin (H2P) to quinones [p-benzoquinone (Q) and naphthoquinone (NQ)] in dimethyl sulfoxide (DMSO). The tetrabutylammonium cation (NBu4+) is also effective to accelerate the electron-transfer reduction of Q and NQ in dichloromethane (CH2Cl2). The hydrogen bonding interaction between the semiquinone radical anion (Q(.-) or NQ(.-)) and NH4+ was confirmed by the ESR spectra of the Q(.-)/NH4+ and Q(.-)/(NH4+)(2)[NQ(.-)/NH4+ and NQ(.-)/(NH4+)(2]) complexes in DMSO. Accelerating effects of NH4+ in DMSO and NBu4+ in CH2Cl2 on the rates of photoinduced electron-transfer reduction of Q by HP result from the positive shift of the E-red value of quinones together with a constant E-ox value of H2P 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.