A modified correction-factor analysis is introduced in present study to develop equations 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 that 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 a double-pass device.