Metal mobility in soils and sediments is dependent not only on the well-known physico-chemical parameters, but also on much less well characterised biological parameters. Iron minerals, ubiquitous in the soil mineral matrix, provide preferential supports for trapping heavy metals and metalloids. This work consisted in studying factors favouring the short-term mobility of arsenic by analogy with the biogeochemical reactions occurring in the soils. Incubation experiments with hydrated iron hydroxides spiked with arsenic(V) carried out in vitro in an anoxic environment in the presence of Fe-reducing bacteria (FR) revealed a chemical mechanism (phosphate/arsenic exchange) that causes rapid solution of the arsenic and a biological reducing mechanism in the solubilisation of the iron and arsenic. In the first instance, the bacteria develop and reduce the Fe(III) to Fe(II), which is solubilised; no solubilisation of arsenic was observed during this phase. Next, the concentration of dissolved iron diminishes and the As(V) is reduced to As(III). Some samples showed the presence of vivianite Fe 3 (PO 4 ) 2 .8H 2 O, which results from the precipitation of soluble iron with the phosphate ions present in the culture medium.