Abstract: Three montmorillonites and a nontronite were reduced by sodium dithionite to obtain different amounts of Fe2+ in their octahedral sites. The mass ratio of water to clay, mw/mc, was determined as a function of Fe2+ at several values of the swelling pressure, II. The value mw/mc decreased markedly with increasing Fe2+ at each value of II for each clay. Moreover, curves of II vs. mw/mc for the different clays were displaced downwards as Fe2+ increased. A straight line was obtained when 1n(II + 1) was plotted against 1/(mw/mc); however, at some oxidation states of three of the samples this line exhibited a sharp break at a specific value of 1/(mw/mc). The slope of the line decreased for each clay as Fe2+ increased, and an increase in Fe2+ was accompanied by an increase in the cation exchange capacity. These observations are thought to be due to a collapse or partial collapse of the superimposed clay layers resulting from the increase in cation-exchange capacity.