By means of pulse radiolysis, bromine atoms were generated during one-electron reduction of 1,2-dibromoethane in water. Rate constants of hydrogen abstraction reactions between the bromine atom and several hydrogen donors, RH, were determined using Br- and promethazine as monitor substances. Comparison of the aqueous rate constants with those measured in acetonitrile and some alcohols reveals the hydrogen abstraction rate to be largely unaffected by the polarity of the solvent. In particular, the reaction does not utilize the potential rate enhancement that would result were the free energy gain of HBr dissociation in water to lower the transition state. Thus, the rate-determining step of the reaction seems to produce molecular HBr. For a number of oxygen-containing organic substrates, the aqueous rate constants were found to display an excellent linear correlation with the equilibrium constants of the hydrogen abstraction reactions. However, at similar thermochemistry, the rates for alkanes are significantly lower than those for alcohols and similar heteroatomics. This was interpreted in terms of a polar transition state, where the ionization potential of the radical, R(.), is an important parameter. During one-electron reduction of 1,2-dibromoethane by the hydrated electron, some Br-2(.-) was found to form even in the presence of high concentrations of Br-. atom scavengers. This fact was utilized for the determination of the rate constant of decay of Br-2(.-) into Br-. nd Br- (1.9 X 10(4) s(-1)). By means of this rate constant, the equilibrium constant for Br-. + Br- reversible arrow Br-2(.-) was revised to 6 X 10(5) M(-1), resulting in E degrees(Br-./Br-) = 1.96 V vs NHE.