Pulse radiolysis experiments were performed on two equilibrium reactions, e(aq)(-) + H+ reversible arrow H (1) and e(aq)(-) + NH4+ reversible arrow H + NH3 (5), in aqueous solution in a range between 25 and 250 degrees C. Equilibrium 1 was observed at temperatures above 100 degrees C, where dissociation of H becomes rapid, while equilibrium 5, which had been studied by Schwarz below 100 degrees C, was observed in the whole temperature range. The two sets of measurements gave consistent results. pK(a)(H) was found to decrease from 9.59 +/- 0.03 at 25 degrees C to 6.24 +/- 0.09 at 250 degrees C, while its van’t Hoff plot curved concave upward to show that Delta C-p degrees(1) becomes increasingly more positive above 100 degrees C. The curvature is qualitatively as expected from decrease in the stability of ionic species with reduction in the dielectric constant of water. C-p degrees(e(aq)(-)) at 25 degrees C was estimated to be -4 +/- 11 cal K-1 mol(-1), which is large as an anion, probably because of loose solvation structure of e(aq)(-). Arrhenius plots of rate constants k(1) and k(5) both showed significant concave upward curvature in a region corresponding to temperature above 150 degrees C, while those of k(-1) and k(-5) did concave downward curvature in the same region. The reducing dielectric constant is presumed substantially to influence these features.