The important characteristics of the cation exchange equilibrium of Na+/H+, namely, the experimental isotherm of Na+, selectivity coefficient, KNa/H;and thermodynamic equilibrium constant, KT,Na/H;have been determined for a 65% crosslinked, porous, strongly acidic cation exchanger, XP-051-RE. The experimental isotherm of Na in a binary system of Na+/H+exchange was determined using the tracer-pulse technique. A concave-upward isotherm was observed for the exchanger, that is Na+is less preferred than H+. The selectivity coefficient, KNa/H;varies with the concentration of Na+