The aim of this study is to evaluate the influence of organic matter on arsenic removal by coagulation/flocculation on both a model water with low mineral content and a natural water sample. Ferric chloride was used as coagulant at concentrations avoiding the preoxidation step usually required to oxidize As(III) and increase its removal. Arsenic removal was accomplished by combining evaluation of arsenic residual concentrations and speciation analysis with zeta potential measurements. A preliminary study evaluated the influence of coagulant dose, coagulation pH, and organic matter on As(III) and As(V) removal. The main conclusions were: (i) As(III) removal depended on coagulant dose and on the number of sites available on hydroxide surfaces rather than on coagulation pH; (ii) As(V) removal depended on the zeta potential of colloidal suspension and was more influenced than As(III) by coagulation pH and the presence of organic matter: (iii) organic matter removal followed As(V) removal. This allowed determination of adsorption as the main mechanism occurring during As(V) and organic matter removal and supposing precipitation/coprecipitation as an important As(III) removal mechanism. Adsorption on preformed ferric hydroxide flocs experiments allowed then confirmation of these hypotheses. (C) 2009 Elsevier Inc. All rights reserved.