The main oxirane-ring opening reactions that occur during the manufacture of epoxidized vegetable oils using a strongly acidic, gel-type ion exchange resin (IER) (Amberlite IR-120, 8% crosslinking) were analyzed. By using a heterogeneous liquid-liquid-solid model, the reactivity on each phase was individually assessed, taking into account each transport step as well as the relevant partition equilibria. The combined results on the attack onto the oxirane ring of epoxidized vegetable oils by either hydrogen peroxide (H2O2) or solvated acetic acid (AA) indicate that under 'regular' process conditions these attacks proceed in the kinetic regime; that is, they are not mass-transfer controlled. The study indicated that most of the degradation occurs on the catalyst and, also, confirmed that the external surface protons of the IER are the main responsibles of the deleterious degradation of the oxirane ring as, in both cases, the degradation rate was directly proportional to the available external area of the catalyst. (c) 2006 Elsevier B.V. All rights reserved.