The chemical state of Fe in standard humic substances (HS) was determined by X-ray absorption fine structure. Iron(III) complexed with HS, which can inhibit complexation of other metal cations with HS, is the dominant form of Fe in HS. The degree of inhibition of complexation by Fe-III was evaluated using a model for complexation between various metal cations and HS. Our results suggest that contribution of Fe must be considered in the evaluation of complexation between metal cations and HS.