An effect is studied of all the individual mass transport mechanisms on the overall mass fluxes during membrane extraction, i.e. for the process that is both energy efficient and easy to operate. In multicomponent systems each of the constituent mechanisms is defined by an appropriate matrix of mass transfer coefficients. The method developed to assess the relative importance of the individual transport mechanisms is validated using multicomponent extraction, studied experimentally by Kubaczka, Burghardt and Mokrosz (1998, Membrane-based solvent extraction in multicomponent systems. Chemical Engineering Science, 53, 899-917), as a test case. If resistances to mass transfer, identified by the method as insignificant, are omitted from the simulations, the agreement between numerical predictions and experiment is not impaired. It is also demonstrated that the accuracy of calculating mass fluxes in membrane extractors became worse when the variation of diffusivities with concentration along the diffusion path is neglected, and also, when the effect of the membrane on interrelationships between the mass fluxes (referred to the stationary system of coordinates) is not taken into account.