Chromate adsorption on amorphous iron oxyhydroxide was investigated in dilute iron suspensions as a single solute and in solutions of increasing complexity containing CO2(g), SO4S (aq), H4SiO4(aq), and cations (K , MgS , CaS (aq)). In paired-solute systems (e.g., CrO4S -H2CO3*), anionic cosolutes markedly reduce CrO4S adsorption through a combination of competitive and electrostatic effects, but cations exert no appreciable influence. Additionally, H4SiO4 exhibits a strong time-dependent effect: CrO4S adsorption is greatly decreased with increasing H4SiO4 contact time. In multiple-ion mixtures, each anion added to the mixture decreases CrO4S adsorption further. Adsorption constants for the individual reactive solutes were used in the triple-layer model. The model calculations are in good agreement with the CrO4S adsorption data for paired- and multiple-solute systems. However, the model calculations underestimate CrO4S adsorption when surface site saturation is approached. Questions remain regarding the surface interactions of both CO2(aq) and H4SiO4. The results have major implications for the adsorption behavior of CrO4S and other oxyanions in subsurface waters.