The electron transfer (ET) reaction from the triplet state of 1,4-dimethoxybenzene ((DMB)-D-3*) to the hydronium ion (H-aq(+)) in aqueous solution at 295 K was studied by means of the laser photolysis method. (DMB)-D-3* was quenched by H-aq(+) and D-aq(+) with the quenching rate constants of 7.7 (+/-0.4) x 10(6) and 2.6 (+/-0.4) x 10(6) M-1 s(-1), respectively. The occurrence of the ET reaction was substantiated by formation of a DMB cation radical (DMB+.) coinciding with disappearance of (DMB)-D-3*. From kinetic analyses, the rate constant (k(et)(H)) for the ET reaction from (DMB)-D-3* to H-aq(+) was determined to be 7.5 (+/-0.4) x 10(6) M-1 s(-1). The observed ET rate exhibited a significantly smaller value compared to the diffusion-controlled rate in aqueous solution. The free energy changes for the ET reaction were estimated to be +28 and -175 kJ mol(-1) for the reactions (DMB)-D-3* + H-aq(+) --> DMB+. + H and (DMB)-D-3* + H-aq(+) --> DMB+. + 1/2H(2), respectively. The observed small ET rate constant (7.5 (+/-0.4) x 106 M-1 s(-1)) revealed that the ET corresponds to the former reaction. The large isotope effect (k(et)(H)/k(et)(D) = 3.0) was observed for the ET reaction between (DMB)-D-3* and H-aq(+) (or D-aq(+)). The fluorescence of DMB was also quenched by H-aq(+) with a quenching rate constant of 6.2 (+/-0.4) x 10(7) M-1 s(-1). The singlet quenching was similarly ascribable to the ET reaction from (DMB)-D-1* to the hydronium.