The adsorption behavior of As(III) as a function of pH on an iron oxide impregnated activated carbon (FeAC) at different adsorbate/adsorbent concentrations was modeled using the surface complexation modeling approach (SCM). The surface complexation constants developed from single sorbate experiments successfully predicted competition between As(V) and As(III) and the SCM predictions were verified experimentally. The monoprotic surface site representation described the experimental data better than the diprotic representation. Based on surface complexation modeling simulations, the effect of As(V) on As(III) removal was greater than As(III) on As(V) removal. As(III)(As(V) was observed for the FeAC and the virgin carbon beginning at pH=8.