The speciation of aqueous and adsorbed As forms of arsenic (As) is a major environmental concern in the presence of humic acid (HA). The speciation during As adsorption process by the effect of contact order were evaluated in various equilibrated ternary systems consisting of As, HA and hematite. One ternary system was composed of the preequilibrated As(III)- or As(V)HA complex, with the subsequent addition of hematite ((As-HA)-hematite system), and the other consisted of the preequilibrated HA-hematite, with the addition of As(III) or As(V) (As-(HA-hematite) system). The presence of HA led to a decrease in the As adsorption, opposite to cationic adsorption. The order of the amounts of As adsorption were found to follow as: As(V)-hematite > hematite-(As(V)-HA)> As(V)-(HA-hematite)> As(III)-hematite > hematite-(As(III)-HA)> As(III)-(HA-hematite). Free As(V) and As-HA complex were preferentially adsorbed onto the hematite surface. The immobilization of As can come from adsorbed HA on mineral surfaces, and formation of As-HA complex, following their slow kinetics. (c) 2006 Elsevier B.V. All rights reserved.