Rate constants for chemical reactions in various aqueous systems have been measured and analysed. Oxidations of a number of organic dyes by the compound sodium perborate tetrahydrate have been extensively studied kinetically. These rate constants are compared to those for hydrogen peroxide and sodium percarbonate, for similar oxidations under the same conditions. A general review of peroxoanion chemistry is included, and a computer simulation of the perborate species in solution is presented. The theories of Debye and Huckel concerning kinetic salt effects are examined in the light of Desnoyer's theory of co-sphere interaction for ions in solution. The effects of added salt on the alkaline hydrolysis of the disodium salt of bromophenol blue are reported and analysed using Pitzer's equations for the activity coefficients of single ions in aqueous salt solutions. The Kirkwood-Buff theory of interactions in binary aqueous mixtures has been examined and extended according to the methods proposed by Ben-Naim. Calculated Kirkwood-Buff parameters are presented for ethanol + water, and t-butanol + water mixtures. Theory is described for the calculation of the so-called gk parameters for first- and second-order reactions. These parameters have been calculated for the alkaline hydrolysis of an iron(II) complex, and the spontaneous hydrolysis of 4-methoxypheny1-2, 2-dichloropropionate in t-butanol + water mixtures. The significance of the calculated parameters, in terms of the role of preferential solvation in the reaction mechanism, is discussed.