In this study we evaluated the efficiency of red muds (RM, a bauxite residue) to immobilize the arsenic present in a polluted sub-acidic soil (UP-soil: total As 2428 mg kg(-1)) and to influence some chemical, biochemical and microbiological properties after 2 years since RM addition. The RM addition caused a pH increase, a striking decrease of total organic carbon and a significant increase of water-soluble C. N and P. The analysis of As mobility through sequential extraction showed a reduction of the water-soluble arsenic in the RM-soil compared to the UP-soil (3.44% and 5.59% of the total As respectively) and a remarkable increase of the residual (non extractable) arsenic fraction in the RM-soil (>300% compared to UP-soil). RM addition increased significantly the microbial abundance and the activity of selected enzymes (dehydrogenase, urease) with respect to UP-soil while had a major influence on the structure of soil microbial communities as evaluated by the Biolog Community Level Physiological Profile. The reduced As mobility, together with an increase of C, N and P labile-pool (likely originating from a "de-structuring effect" of RM on the soil organic matter) were identified as the key factors affecting the biological activity in the RM-treated soil. (C) 2010 Elsevier B.V. All rights reserved.