A systematic study with the negatively charged cation-exchange resin Amberlite 200 was performed to investigate the influence of anion type on cation exchange. A comprehensive data set was obtained for binary and ternary cation exchanges involving Na, Mg, and Zn on a strongly acidic, sulfonate cation-exchange resin in electrolyte systems with the single anions chloride, perchlorate, and sulfate, and with 1:1 mixtures of them at two total cation concentrations of 0.05 and 0.2 N. The binary and ternary cation exchange data exhibited internal consistency in that Mg and Zn were selected over Na to a comparable degree. The binary and ternary cation exchange data also indicated clearly an effect of anion type, with reduction in selectivity of Mg and Zn relative to Na in the presence of more strongly complexing anions such as sulfate. The effects of anions in cation exchange may be ascribed in part to their association with cations in solution, as demonstrated by application of conventional mass action models for ion exchange with consideration of solution-phase speciation. The ability of mass action models including solution speciation to predict binary and ternary cation exchange in different anion backgrounds based on model parameters extracted only from data obtained in chloride background was also tested. The Gaines-Thomas model provided the best predictions of binary and ternary cation exchange on the sulfonate resin for the different anionic media and ionic strengths. Mass action models can account at least approximately for the effect of anion type on cation exchange if solution speciation and activity correction are properly considered.