An extended mass transfer model for pervaporation that includes upstream, membrane and downstream resistances is presented. Included is a factor that can account for the potentially adverse change in the partition Coefficient with low activity. Concentration-independent membrane diffusion coefficients have been assumed and so the model is mainly restricted to organophilic pervaporation. The desorption factor can be important for the permeation of less volatile organics. It is shown that the separation factor for pervaporation is influenced by the product of four terms: vapour-liquid equilibrium separation factor (alpha(VLE)), intrinsic membrane properties (E-mem*), effectiveness of the module (8(bl))and the operating conditions (including feed concentration and permeate pressure). A quadratic expression relating inter alia the composition of each phase, membrane resistance and saturated vapour pressure was obtained. The effect of downstream pressure on partial fluxes and separation factor was investigated and a comparison between pervaporation and gas separation is included. Pervaporation experimental data was also used to test the derived model and found to be in good agreement. Simple dimensionless terms E and y(i)* were derived to characterise the transport performance at various permeate pressures. The E number is a ratio of the organic flux per unit of driving force to water flux per unit of driving force determined at ultimate vacuum, y(i)* is the permeate mole fraction of the organic at ultimate vacuum. Using E-y(i)* coordinates a useful diagram, similar to Geldart's powder classification for fluidisation, was developed for the characterisation of the behaviour of pervaporation systems. The effect of various factors, including temperature, feed concentration and boundary layer resistance, was investigated and general rules for predicting the change of behaviour developed.