Some agricultural soils was historically polluted by the destruction of chemical shells. The present work used laboratory experiments aiming to determine the influence of water saturation and organic substrate supply on the speciation and mobility and speciation of arsenic (As). A column filled with soil was fed with synthetic water, transiently supplied with organic substrates, during 320 days. Important quantities of As were leached. The initial As(V) present in soil was reduced to As(III) during the periods of organic substrates supply. Most of the As initially present in the solid phases was reduced at the end of experiment. Active Astransforming microbes were quantified in the solids and the evolution of their relative concentrations seemed to be linked to As speciation. The proportion of aioA genes encoding an arsenite oxidase, expressed as the ratio of aioA/16S genes abundances, tended to increase with the proportion of As(V) in the solids. The microbial activities played a major role in the evolution of As speciation, probably influencing As mobility.