1,4,7-Trimethyl-1,4,7-triazacyclononane (TMTACN), MnSO4 and H2O2, in basic aqueous acetonitrile, is an effective system for the epoxidation of cinnamic acid. The influence of a selection of organic additives, potential co-ligands for the manganese species, on the reactions has been studied by UV-vis spectroscopy and ESI-MS. The mechanism of the most efficient system, with added oxalic acid, has been investigated in more detail using cinnamic acid and seven of its 3- or 4-substituted derivatives. A Hammett correlation of rate data shows that the active oxidant is electrophilic (p value -0.63). Oxygen (O-18) labelling experiments reveal that H2O2 and not H2O is the source of the oxygen in the epoxide. Possible mechanisms for the reactions are discussed. (C) 2004 Elsevier B.V. All rights reserved.