The decomposition reaction of peroxovanadium species, either formed in situ by addition of H2O2 to vanadium compounds or added as the isolated complex VO(O2) (Pic)(H2O)2 (Pic = picolinic acid anion), has been studied both in aqueous acid solutions and in CH3CN. In all cases, evidence for a radical chain mechanism has been obtained, initiated by the reaction between two molecules of peroxide, either H2O2 or a monoperoxovanadate. The occurrence of vanadium and other intermediate with unpaired electrons have been observed by indirect but rather convincing kinetic evidence.