Rate constants for chemical reactions in various aqueous and micro- heterogeneous systems have been measured and analysed. A large part of the thesis is concerned with interpreting kinetic trends in two water-in-oil microemulsions. Kinetic data for inorganic reactions involving low-spin iron (II) di-imine oorplexes and for reactions involving the organic substrates, 2,4-dinitrochlorobenzene, crystal violet and malachite green in microemulsions have been collected and used to identify different structural zones in these solvent systems. The kinetic data are interpreted in terms of a model in which reaction occurs at an interfuse. Reactions involving cis-bis(pyridine)tetracarbonylmolybdenum (0) in oil- in-water, water-in-oil microemulsions and organic solvent mixtures have been studied. Aquation reactions of iron (II) hexadentate Schiff base have been used to identify microheterogeneous phases in water-rich '2-butoxyethanol + water' mixtures. Solvent effects on initial and transition states are discussed for reactions of iron (II) glyoxal bis-N methylamine with hydroxide ions in 'methanol + water' mixtures and reactions of iron(II) 1,10-phenan- throline with hydroxide ions in 'isopropanol + water' mixtures. Transfer chemical potentials for single ions, in 'acetone + water' mixtures, have been estimated, using solubility data for salts in conjunction with the assumption that transfer chemical potentials for tetraphenylboronate and tetraphenylarsonium ions are equal. Effects of added salt on the neutral hydrolysis of phenyldichloro- acetate in aqueous solutions are examined in terms of solvent cosphere interactions between ions. The temperature dependence of rate constants for reactions of iron (II) 4-methyl, 1,10-phenanthroline have been examined. The results are discussed in terms of isobaric, isothermal and isochoric activation parameters. The meaning of the term 'isochoric' is clarified in this context.