Arsenic mobilization in soils is mainly controlled by adsorption/desorption processes, but it may be also coprecipitated with aluminium and/or iron in natural environments. Coprecipitation of arsenic with aluminium and iron oxides are practical and affective treatment processes for removing arsenic from drinking waters. Studies on the nature, mineralogy, reactivity of Al-As (and Fe-As) coprecipitates formed at different pH and Al/As molar ratios (R) as well as the possible removal of arsenic from these samples have received scant attention. Aluminum-arsenate coprecipitates were synthesized at pH 4.0, 7.0 or 10.0 and R of 0, 0.01 or 0.1. The suspensions were aged 1 or 7 months at 50°C and the mineralogy, chemical composition, surface properties (surface area and pzc) of the freeze-dried precipitates were studied. The sorption of phosphate and the removal of arsenate from the Al-As coprecipitates have also been studies. In the absence of arsenate gibbsite (pH 4.0 or 7.0) and bayerite (pH 10.0) formed, whereas in the presence of arsenate very poorly crystalline boehmite (pseudoboehmite) formed at pH 4.0 (R = 0.01 and 0.1), 7.0 and 10.0 (R = 0.1) and did not convert into Al(OH)3 polymorphs even after 7 months of aging at 50°C or longer. The surface properties and chemical composition of the Al precipitation products were affected by the initial pH, As/Al molar ratio and aging. The aluminum-arsenate coprecipitates were solubilized by 6M HCl or oxalic acid/oxalate solution at pH 3.0 and the amounts of aluminum and arsenic present into solutions were determined. Oxalic acid/oxalate solution at pH 3.0 was not selective in solubilizing poorly crystalline boehmite (as for ferrihydrite), being able to solubilize no more than 70% of precipitates containing only short-range Al-oxyhydroxides. Arsenate appeared to be present within the network of short-range ordered boehmites and/or on the external surfaces of the precipitates. The adsorption of phosphate onto Al precipitates was influenced by the mineralogy of the samples, their surface properties and the amounts of arsenate present in the precipitates. Arsenate inhibited and/or retarded phosphate adsorption. Large amounts of phosphate partially replaced arsenate present into the precipitates formed at R = 0.1. The quantities of arsenate desorbed by phosphate increased with time. The percentages of arsenate desorbable by phosphate was always lower than 30% of the amounts present in the materials.