Four industrial by-products (phosphogypsum, PG: red gypsum, RG; sugar foam, SF and ashes from the combustion of biomass, ACB) were evaluated as possible amendments for reducing the leachability and bioavailability of As and Se in a metalloid-spiked acidic soil. The treatments were applied as single, double and triple amendments and at two different rates. The effectiveness of the treatments was evaluated after a series of leaching experiments using a chelating agent (DTPA solution) or a weak acidification (acetic acid at pH 4.93). The most effective treatments (ACB and RG, both applied at high rate) were identified by means of Cluster Analysis using the leachability indexes. Different sorption mechanisms involved in the overall reduction of metalloid leachability were identified using scanning electron microscopy (SEM-BSE and SEM-EDS). In the ACB-treated samples, Se was found associated to organic matter aggregates and to Fe compounds. In the RG-treated samples, EDS analyses showed that As and Se were associated to Fe/Ti (hydr)oxides phases which are present not only in the by-product as maghemite and rutile, but also in the soil as hematite and goethite. In addition, the application of RG induced the formation of non-crystalline Al-hydroxy polymers with As and Se in their composition. (C) 2009 Elsevier B.V. All rights reserved.