A modified correction-factor analysis is introduced in the present study to develop the theory for estimating the mass-transfer rate in multipass backward-flow membrane extractors, in which the unknown solute outlet concentrations are neither in the estimating equation nor in the modified correction-factor charts, and thus the calculation is easy and direct without using the try-and-error method, which should be employed in the conventional correction-factor analysis. Experimental results confirm the prediction values for the extraction of acetic acid from aqueous solution by methyl isobutyl ketone in the double-pass device. The fact that the desirable effect of the increase in mass-transfer coefficient by applying the multipass operation compensates for the undesirable effect of the enhancement of hydraulic-dissipated energy, has been justified.