Waste waters from the essential oils industry contain terpenes which have to be recovered, and pervaporation proved to be an efficient method for that purpose. In order to identify the mass transfer limitations during the pervaporation of two particular solutes, linalool and eucalyptol, experiments were performed with different thicknesses of polydimethylsiloxane membranes under isothermal conditions. Mathematical treatment based on a resistances-in-series model shows a good agreement between experimental data and theory. Moreover it can be shown also that under certain conditions, the limiting step can result from the transfer in the liquid phase, even though in most cases of pervaporation, the limitation lies on the transfer within the polymeric material. Estimation of the diffusivities of the two terpenes in the polymer can be done from this treatment.