Ion exchange isotherms at 298 K describing the uni-trivalent exchange of LiY, NaY, KY, RbNaY and CsNaY with Ce 3+ at total normality 0.1 mol equiv. -1 are presented. The out-going cation has a considerable influence on the progress of exchange and the overall preference of the zeolite phase for the entering Ce 3 + ions decreases in the order LiY > NaY > KY > RbNaY > CsNaY. The maximum level of cerium exchange is greatest in the Ce/K-Y system and increases with increasing temperature, in the range 298 K < T< 373 K, as the hydration sheath is stripped from the cerium ions allowing access to small cages and exchange with the potassium ions located therein. Isotherm reversibility is examined by back exchange of the maximally exchanged CeY zeolite. The marked preference of the zeolite phase for the entering cerium ions is illustrated by employing normalized selectivity coefficients. The relationship between cerium loading and selectivity is fitted to third order polynomial equations and the polynomial coefficients and the error of the fits are presented. Such considerations as site heterogeneity, ion location and selectivity reversal are considered. Values of the equilibrium constant and standard energy changes for the five exchange systems are tabulated. The variation of the activity coefficients of the exchanging ions in the zeolite phase with cerium loading is illustrated, application of the triangle rule is addressed and the enthalpy and entropy changes associated with the cerium exchange of KY are provided.