This work describes utilization of the multivariate calibration method PLS (partial least squares) in the investigation of K+ transport through a choloroform membrane by using mixtures of three isomeric organic anions as counterions. A comparison between the results obtained by this method and those obtained by the stepwise multiple linear regression method was also made. The BLM (bulk liquid membrane) method was employed in the transport by using a neutral crown ether, 18-crown-6 (18C6) as the carrier. Multivariate calibration was necessary since there are superpositions among the UV-Vis spectra of the three isomers. Univariate methods are shown to be unable to quantify samples in these conditions.