Wetting and drying experiments performed at 60 °C were used to examine the effect of various concentrations of NaCl solutions on the degree of K-fixation. Cheto and Texas montmorillonites were used as a starting material for experiments. K-saturated samples were put through up to 100 cycles in 0.05–5 M NaCl solutions. The expandability values, measured by XRD peak position method, indicate a higher degree of K-fixation in the most diluted NaCl solution at a given number of cycles in comparison with smectites wet by distilled water. However, the effect of NaCl solutions of higher molarity on the degree of K-fixation is not straightforward. Smectites wet by moderately concentrated NaCl solutions approach the K-fixation pattern of water-wet smectites. K-fixation in the smectites in the most concentrated NaCl environment was clearly retarded. The effect of NaCl on K-fixation was much less visible when a buried altered volcanoclastic sample was used.