The use of solvent-free systems in the oil and fats industry is commonplace. Initial studies on interesterification were carried out in solvent systems because the lipase was immobilized solely by adsorption onto particles of diatomaceous earth. In this study, the mass transfer characteristics associated with the continuous interesterification of olive oil in a solvent-free system have been examined, for lipase immobilized on the three ion-exchange materials : Duolite ES562, Duolite ES568, and Spherosil DEA. The process of immobilization is influenced by the internal structure of the material and this in turn influences the interesterification activity of the catalyst. Individually prepared catalysts for the three support materials have shown that external mass transfer limitations are unlikely even at low flowrates. In the case of Spherosil DEA, with a mean pore diameter of 1480 angstrom, the wide pores would be expected to reduce internal mass transfer limitations; however, it is more likely that the reduction in activity with increased catalyst loading is due to the lipase molecules being immobilized in a tightly packed monolayer. In such a situation, some active sites of the lipase molecules would become inaccessible to substrate molecules leading to an observed reduction in activity. For Duolite ES568, the observed results are very similar to those seen for Spherosil DEA, however, the pore structure of this support material indicate that some internal mass transfer limitations may also be occurring. Yet the contribution of the individual effects cannot be determined. The results observed for the support Duolite ES562 are different than those observed for the other materials and reflect the heterogeneity of Duolite ES562. The large proportion of narrow pores in the support mean that, for the catalysts examined, immobilization is most likely to have occurred in the external pores of the particles, and as such no internal mass transfer limitation is observed. It is clear that for interesterification the material chosen for enzyme immobilization will have an important role in determining the catalyst efficiency. External mass transfer limitations are very minor and observed internal mass transfer limitations may be caused by both internal mass transfer and the manner in which the immobilization process occurs.