Abstract: The effect of Fe oxidation state on the specific surface area, Sm, of nontronite was studied using the <2-µm, Na+-saturated fraction of the SWa-1 and Garfield nontronite reference clays. The reduction of structural Fe3+ in the octahedral sheet of the nontronite decreased Sm as measured by the adsorption of 2-ethoxyethanol (ethylene glycol-monoethyl ether, EGME). The swellability in water of the nontronite also decreased during reduction. The amount of nonexchangeable Na+, on the other hand, increased with increasing Fe2+ content and was highly correlated with EGME adsorption (r = -.985). The relationship between Sm and Fe2+ was attributed to the collapse of partially or fully expanded layers to unexpanded layers.