Sorption isotherms for the ternary system Cs-Sr-Co using ’Nonscents,’ a zeolitized volcanic tuff material from southeast Arizona, as sorbent were measured with a columnar flow technique. The isotherms were correlated with an ion-exchange phase-equilibrium model, and the triangle rule of ion exchange has been validated. Sorbent-phase selectivity for the ternary system has been successfully predicted from a knowledge of the selectivity of constituent binary pairs. The study helps in the understanding of multisolute sorption behavior during host rock-ground water interactions relevant to a high-level nuclear waste repository.