The distribution coefficients of Cs in demineralized water and sea water were proportional to the ratio of either exchangeable K to total K or exchangeable Mg to total Mg of the marine sediments. The degree of the desorption was, on the contrary, inversely proportional to these ratios. The linear relation held even if the samples of the same location were concerned. Mass action law could apparently be applied to Cs-K and Cs-Mg systems, when Cs was employed at the tracer concentrations. The observed separation quotients for the 2 systems were abnormally higher than the equilibrium constants at the macroconcentrations. Equilibrium constant increases in the order: Cs-K, Cs-Na, Cs-Mg and Cs-Ca.