The quantitative description of synergistic solvent extraction systems is usually a difficult problem in equilibrium analysis because a relatively large number of species can be involved. Both sophisticated methods of interpretation and extensive experimental data are required. The graphical methods of “slope analysis”and “continuous variation”provide useful guidance only when the minimum number of species is present and especially when there are no competing reactions between the two synergistic extractants. A general least squares computer program (SXLSQ)is described to deal with the general case in which many possible species must be considered. It will accept six different kinds of equilibrium data for a given system in a single data set and can be used interactively at a terminal. Analysis of data on the extraction of Mn(II) by an organophilic sulfonic acid in combination with each of two crown ethers showed that a unique identification of the species formed cannot be made without additional information about the degree of association of the sulfonic acid in the organic diluent. But narrow limits could be placed on ratios of the integers defining the principal species formed.