The Cerium(lV) ion has been shown to exist in perchloric acid solution as a mixture of Ce4+ and CeOH3+ The hydrolysis constant has been determined spectrophotometrically and compared with the existing data on tetravalent ions. From a spectrophotometric study, values of the molar extinction coefficients of each ion have been determined over a wavelength range of 300-400 nm. Two different types of apparatus utilizing the stopped-flow technique have been used to study the following reactions: a) the cerium(lV) oxidation of alpha-hydroxy acids, malonic acid, L-cysteine and DL-penicillamine b) the reaction of thiomalic acid with thalliuim(III) and c) the vanadium(v) oxidation of thiourea. Ceriuro(lV) has been shown to oxidise alpha-hydroxy acids (HI) in perchlorate media via well defined intermediate complexes. The ultraviolet spectra and the equilibrium constants of these complexes have been determined and a comparison made with the kinetically derived values and compared with other oxygen containing substrates.