Methyl oleate epoxidation with a peroxophosphotungstate (POW) catalyst was performed under solvent free conditions After underlying a synergistic effect of hydrogen peroxide and the dioxygen flow to obtain a total epoxide yield new investigations were realized to understand this effect The POW catalyst mechanism was studied using labeled oxygen flow and radical traps We concluded that the reaction occurs in two parallel ways a major one thanks to a concerted mechanism and a minor radicalar one leading to the formation of methyl stearate alcohol and diol intermediates Oxygen flow is only necessary to perform the radicalar way (C) 2010 Elsevier BV All rights reserved