Competitive adsorption of molybdate, sulfate, and selenate onto gamma -Al2O3 was investigated in the present study. Binary solute systems of MoO42- + SO42-, MoO42- + SeO42-, and SO42- + SeO42- and a ternary solute system of MoO42- + SO42- + SeO42-were evaluated to determine their relative effects on competitive adsorption on the gamma -Al2O3 surface. Anionic competitive adsorption efficiency was pH dependent. The higher the pH, the lower the efficiency of MoO42- preventing SO42- and SeO42- adsorption; similar results were found in SeO42-depressing SO42- adsorption. This research found that more sites are occupied in mixed anionic adsorbate systems than when either ion is present alone. The results suggest that the gamma -Al2O3 surface is composed of many groups of binding sites. Because of the heterogeneity of adsorption sites, the triple-layer model (TLM) predicted the competitive effects qualitatively but not quantitatively. TLM gave reasonable descriptions of molybdate adsorption in the presence of sulfate and selenate, indicating that the model may be useful in predicting molybdate adsorption on gamma- Al2O3.