Abstract: Chromate (CrO42-) adsorption was investigated on kaolinite (0.2–2 µm) saturated with NaClO4 over a range of pH. Adsorption increased with decreasing pH because of protonation of chromate and/or variable charge sites on kaolinite. Chemical pretreatment to remove noncrystalline and crystalline oxide contaminants affected the magnitude of CrO42- adsorption, but not the pH range over which CrO42- adsorbed. Chromate adsorption at different sorbate and sorbent concentrations increased below the pHzpc for the kaolinite edge, suggesting the formation of weak surface complexes. If CrO42- and SO42- were present at equal concentration (5.0 x 10−7 M), the two solutes sorbed independently, suggesting binding to separate sites. The presence of excess SO42- (5.0 x 10−4 M), however, unexplainably enhanced CrO42- adsorption. The adsorption of both chromate and sulfate can be described in terms of a site-binding model of the kaolinite edge, in which the edge is viewed as composite layers of Al and Si oxide. Surface complexation constants for CrO42- on kaolinite were similar to those for alumina, pointing to the importance of Al-OH edge sites in chromate adsorption.