Oxidation of caffeic acid (3,4-dihydroxycinnamic acid) 1H(3) has been studied by electrochemical methods and by pulse radiolysis in aqueous and organic solvents. The results have been compared with the behaviour of 4-coumaric acid 2H(2) and ferulic acid 3H(2). The first oxidative intermediates have been characterised by their UV spectra and oxidation potentials. In the case of 2H(2) and 3H(2), the initial radicals decay by a second order process indicating a radical-radical coupling mechanism. On the contrary, for caffeic acid the oxidation leads to the formation of the corresponding o-quinone through disproportionation of the initial semiquinone radical.