The spectrum of the hydrated electron was determined in the temperature range 5-300 "C by using strongly alkaline solutions and high hydrogen pressure. At temperatures up to about 150 "C the temperature coefficients of E, and AE1/2 are -2.8 X and 2 X lo4 eV K-', respectively. E,, is the energy maximum absorption and AEII2is the half-width of the spectrum. The temperature coefficient of cmaxG is 50 f 10 dm3 mol-] cm-' K-I in the temperature range 5-300 "C. c,,,G at 21 "C is (1.18 i 0.15) X 10' dm3 mol-' cm-' in these solutions. The temperature coefficient of G(e,;) is probably close to zero. The rate constant of the second-order decay (2k) is (1.00 f 0.05) X 1O'O dm3 mol-] s-I at 20 "C, independent of pH. The activation energy of the reaction is 23 f 1 kJ mol-] (5.4 f 0.2 kcal mol-') at temperatures up to 150 "C. The decay at temperatures above 150 "C becomes slower with increasing temperatures but still follows second-order kinetics for 2-3 half-lives. At these high temperatures it is not possible to avoid dissolution of silicon dioxide from the synthetic quartz cell. The silicon dioxide dissolves as silicate, which lowers the pH of the starting solution. As long as the final pH is higher than N 10, the decay rate is still decreasing with temperature, but if the final pH becomes lower than -9 because of dissolved silicate, $e decay rate increases. The decay is independent of the concentration of silicate. Also, silicate even at the highest concentrations does not change the electron spectrum, half-width, or em& to any significant degree at ambient and higher temperatures. -Th e simplest mechanism capable of describing the kinetic data at various temperatures is the equilibrium e,; + e,; F-? (e22-)aq H2 where the dissociation reaction has a higher activation energy than the dimerization reaction. Calculations show that the activation energy of the dissociation reaction is higher than 30 kcal.