Na/Ca ion-exchange isotherms were performed on a Na-saturated montmorillonite for different constant normalities of the aqueous chloride solution and at two solid/solution ratios. The experimental data suggest that the affinity of Na+ for the solid increases with total normality and m/V ratio and that a significant proportion of CaCl+ is also sorbed. A thermodynamic modeling procedure with one sorption site and three sorbed cations (Na+, Ca2+ and CaCl+) was applied to interpret the data. We show that by accounting for the activities of aqueous species, the experimental data obtained at different total normalities for a given solid/solution ratio can be fitted using a unique set of selectivity coefficient values. However, when the m/V ratio is decreased from 25 to 2.5 g/L, an increase in the log Kc(Na+/Ca2+) of up to 0.4 +/- 0.05 was required to interpret the data with a constant log Kc(Na+/CaCl+) value. The same behavior concerning the increase in the log Kc(Na+/Ca2+) with a m/V ratio decrease was observed when using a multi-site model taking into account the specific sorption of H+ The results clearly indicate that the Na+/Ca2+ selectivity coefficients strongly depend on the solid/solution ratio but are independent of the exchanger composition and the total normality of the solution. Such findings provide possible pitfalls when using selectivity coefficients obtained in diluted suspension to assess the sorption in compacted clayrocks and allow a better prediction of the sorption of trace elements in competition with major cations. (c) 2010 Elsevier Inc. All rights reserved.