Interactions of dissolved organic matter (DOM) with clays can significantly affect a variety of soil processes. We studied adsorption and fractionation of hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions of DOM on Cu 2+- and Fe3+-montmorillonite. Adsorption of both samples was higher on Fe3+-montmorillonite than on Cu2+- montmorillonite. A pH increase of about one unit was recorded followed by HoA adsorption by Fe3+-montmorillonite. This suggested exchange of negatively charged DOM groups on surface hydroxyl groups of Fe 3+-montmorillonite surfaces. Adsorption of HoA on Cu 2+-montmorillonite and HoN on Fe3+- and Cu 2+-montmorillonites was governed mainly by van der Waals interactions. Spectroscopic analyses showed a distinct HoA fractionation by molecular size and aromaticity only by Fe3+-montmorillonite. On the basis of the pH measurements (increase in pH following adsorption of acid components) and enhanced DOM fractionation by molecular size and aromaticity we suggest that DOM reacted with Fe3+-montmorillonite similar to goethite.