We report new high-precision measurements of the radiolytic yields (G values) of hydrated electron from pulse radiolysis of dilute H2O and D2O solutions with 15 MeV electrons in the 4-75 degrees C temperature range. Applied radiation dose was calibrated calorimetrically from the change in conductivity of a phosphate buffer solution. The hydrated electron yield was deduced from the yield of stable HCl product produced with various concentrations of methyl chloride scavenger in the presence of 2-propanol to scavenge H and OH radicals. The rate constant for (e(-))(aq) reaction with MeCl in H2O was measured as 4.66 x 10(8) +/- 3% M(-1) s(-1) at 25 degrees C, with an activation energy of (14.24 +/- 0.49) kJ/mol between 3 and 85 degrees C. In D2O the rate constant was found to be 5.25 x 10(8) +/- 3% M(-1) s(-1) at 25 degrees C. Limiting escape yields of (e(-))(aq) at room temperature were found to be G(esc) = 2.50, 2.58, and 2.66 for 2-propanol concentrations of 0.01, 0.03, and 0.10 M in H2O respectively. The temperature dependence of yields was determined for solutions of 0.01 M 2-propanol and 0.12 EA MeCl in both H2O and D2O. The results are described by (t in degrees C G(e)(H2O) = 2.73 + 3.76 x 10(-3) (t - 25) and G(e)(D2O) = 3.24 + 3.32 x 10(-3) (t - 25). By use of these revised yield measurements, previous experiments to determine the hydrated electron diffusion coefficient (J. Phys. Chem. 1992, 96, 199) could be reanalyzed. The refined values differ only marginally from the original report. In a discussion, we apply electron transfer theory to examine which of the very fast reactions of hydrated electrons can be considered "diffusion limited". It is shown that the effective reaction distance for (e(-))(aq) scavenging by nitrobenzene and oxygen changes with temperature as a result of the change in diffusion rate.