Abstract: Shaking of water-soluble fulvic acid with Cu2+-montmorillonite at pH 2·5 increased the interlamellar spacing from 10·0 to 15·1 A. The spacings were measured at ≈0% relative humidity. The extent of interlayer adsorption decreased with increase in pH. Differential thermal analysis and thermogravimetric examinations showed that in addition to interlayer adsorption, fulvic acid was also retained on external clay surfaces. The FA was held so tightly by the Cu2+-clay that > 3/4 of the total amount adsorbed, including degradation products formed from it, resisted decomposition even when heated to 1000°C. I.R. spectra showed that β-diketone groups in fulvic acid reacted with Cu2+ in or on the clay to form acetylacetonate-type chelates. Indications were obtained that the mechanism postulated for the FA-Cu2+-clay interaction may also apply to reactions between FA and montmorillonite saturated with other di- and trivalent metal ions. Reactions of FA in aqueous solutions with metal ions in the presence of clays differ from those in the absence of clay. Montmorillonite appears to affect the conformation of the fulvic acid polymer in a way that favours reactions between C=O groups and metal ions, a type of reaction that has so far not been observed in aqueous solutions in the absence of clay.