Abstract-Three montmorillonites and a nontronite were reduced by sodium dithionite to obtain different amounts of Fe 2+ in their octahedral sites. The mass ratio of water to clay, mJmo, was determined as a function of Fe z+ at several values of the swelling pressure, II. The value mJmo decreased markedly with increasing Fe 2 § at each value of II for each clay. Moreover, curves of II vs. mJmc for the different clays were displaced downwards as Fe 2+ increased. A straight line was obtained when ln(II + 1) was plotted against 1/(mJmc); however, at some oxidation states of three of the samples this line exhibited a sharp break at a specific value of 1/(mJmc). The slope of the line decreased for each clay as Fe 2+ increased, and an increase in Fe 2+ 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.