In multicomponent membrane extraction each of the constituent mechanisms is defined by an appropriate matrix of mass transfer coefficients. The method to assess the relative importance of the individual mass transport resistances has been proposed. This method is based on the calculation of norms of mass transfer matrices and is used to evaluate the relative importance of the individual transport mechanisms in multicomponent extraction studied experimentally by A. Kubaczka, A. Burghal dt, T. Mokrosz, Chem. Eng. Sci. 53 (1998) 899. 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 even when the membrane resistance can be neglected (as was the case in our study), the membrane determines the ratio of mass fluxes in the stationary system of coordinates by defining the coefficients or the bootstrap (transformation) matrix. The analysis reveals that the omission of this effect leads to a visible drop in the accuracy of calculations of mass fluxes in membrane extractors.