Abstract Competitive adsorption of Cd, Cu, Pb, and Zn onto amorphous iron oxyhydroxide (Fe 2O 3 · H 2O (am)) was studied in systems containing pairs of metal adsorbates. The concentration of the more strongly binding metal was 10–100 times that of the other metal in the system. The conditions were such that if the metals competed for the same group of surface sites, adsorption of the weaker-binding metal should have been significantly reduced when the second metal was added to the system. Nevertheless competitive interactions were minimal, indicating that many of the strong binding sites for one metal are not preferred binding sites for other metals. These results support the hypothesis that the Fe 2O 3 · H 2O (am) surface consists of several distinct groups of binding sites. A differential adsorption equilibrium constant is defined and related to the overall average adsorption equilibrium constant. Based on this relationship, the binding constants for a single metal to different surface sites range over at least two orders of magnitude. The differential constant can be used to assess competitive interactions. For instance, when 5 × 10 −5 M Cu and 5 × 10 −7 M Cd compete for sites on Fe 2O 3 · H 2O (am) (1 × 10 −3 M Fe T), less than 2% of the adsorbed Cu binds to “preferred” Cd-binding sites. Failure to account accurately for competitive effects can lead to serious errors when modeling trace metal behavior in natural systems and engineering processes.