Pulse radiolysis experiments were performed on hydrogenated, alkaline water at high temperatures and pressures to obtain rate constants for the reaction of hydrated electrons with hydrogen atoms (H-center dot + e(aq)(-)-> H-2 + OH-, reaction 1) and the bimolecular reaction of two hydrated electrons (e(aq)(-) + e(aq)(-)-> H-2 + 2 OH-, reaction 2). Values for the reaction 1 rate constant, k(1), were obtained from 100 -325 degrees C, and those for the reaction 2 rate constant, k(2), were obtained from 100 - 250 degrees C, both in increments of 25 degrees C. Both k(1) and k(2) show non-Arrhenius behavior over the entire temperature range studied. k(1) shows a rapid increase with increasing temperature, where k(1) = 9.3 x 10(10) M-1 s(-1) at 100 degrees C and 1.2 x 10(12) M-1 s(-1) at 325 degrees C. This behavior is interpreted in terms of a long-range electron-transfer model, and we conclude that e(aq)(-), diffusion has a very high activation energy above 150 degrees C. The behavior of k(2) is similar to that previously reported, reaching a maximum value of 5.9 x 10(10) M-1 s(-1) at 150 degrees C in the presence of 1.5 x 10(-3) m hydroxide. At higher temperatures, the value of k(2) decreases rapidly and above 250 degrees C is too small to measure reliably. We suggest that reaction 2 is a two-step reaction, where the first step is a proton transfer stimulated by the proximity of two hydrated electrons, followed immediately by reaction 1.