Oxidations of unsaturated acid by cobalt(III) perchlorate show the normal kinetic behaviour in being first order with respect to both cobalt(III) and the organic substrate but inversely dependent on the acidity of the solution. Product studies, however, show that two reactions occur concurrently: (i) oxidation via decarboxylation leading to the production of and aldehyde or ketone, e.g. PhCH:CH·CO₂H [graphic omitted] PhCH:CH· [graphic omitted] PhCH:CH·OH [graphic omitted] PhCH₂·CHO and (ii) direct attack on the double bond, i.e. CC + Co³⁺→Ċ–[graphic omitted] + [graphic omitted], which then leads rapidly to double-bond fission, but can in part (e.g. with 2,3-diphenylacrylic acid) involve an intramolecular rearrangement. The product studies have been carried out quantitatively with cinnamic acid, eight of its analogues, and with their aldehydic degradation products by using the small-scale technique previously developed for this work. The approximate rate constants for these oxidations are also listed.