The two-layer surface complexation model combined with the Gouy-Chapman model (SCGCM) was applied to describe and predict experimental data for cation adsorption and exchange on a synthetic resin. Knowledge of the surface properties of the sulfonate macroreticular cation-exchange resin was incorporated in the model. The data set included single, binary, and ternary cation adsorption involving Na, Mg, and Zn in single anionic media of chloride, perchlorate, and sulfate at 0.05 and 0.2 N in total cation concentration (TCC) and in their 1:1 binary mixtures at TCC = 0.05 N. The SCGCM exhibited reasonably good agreement with binary and ternary cation exchanges in the different anionic backgrounds and ionic strengths, based on constants extracted from fits of single cation adsorption data and a small set of binary exchange data. Specific adsorption was predicted to be dominant for the cation adsorption and exchanges on the resin studied in this work. Na was specifically adsorbed on the sulfonate group of the resin to an equal to or greater extent than Mg and Zn, possibly because of reduced surface-binding energy of the divalent cations due to local electrostatic effects on short-range interactions resulting from very high surface charge density associated with the resin. The SCGCM provides insight into the ion-exchange processes and incorporates knowledge of the resin phase, unlike conventional mass action and thermodynamic models. In addition, the effect of electrolyte composition and solution complexation on cation exchange can be described with the SCGCM, allowing consideration of the adsorption of cation-anion complexes. The SCGCM can be applied to multicomponent systems and incorporated in general chemical equilibrium models, but has substantial data requirements for calibration.