The 137Cs sorption and desorption equilibrium between illite and a solution containing 0.5 mM K+ and 1 to 100 mM Ca2+, determined using a kinetic method with direct measurement of the radioactivity in the sorbent solid phase, was attained within 20–25 days. The kinetic curves of the 137Cs sorption onto/desorption from illite, normalized on the maximal sorbed radioactivity, almost coincided at different Ca2+ concentrations. An increase in the 137Cs distribution coefficient (K d) with a decrease in the Ca2+ concentration did not exceed 60% and was due exclusively to the influence of the solution ionic strength. With an increase in the Ca2+ concentration from 1 to 100 mM, the fraction of 137Cs extracted with 1 M CH3COONH4 (αEx) decreased from 24.1 to 14.8%.