Methylation and demethylation of arsenic may change substantially the toxicity and mobility of arsenic in soils. Little is known about demethylation of organic arsenic species in organic soils. We incubated dimethylarsinic acid (DMA) and arsenobetaine (AsB) in soils and aqueous soil extracts from a forest floor and fen, in order to investigate demethylation processes. Incubations were conducted at 5°C in the dark under oxic or anoxic conditions. Arsenobetaine demethylated rapidly in all soil extracts with half-lives of 3.6–12 days, estimated from first order kinetic. Demethylation of DMA was relatively slow with half-lives of 187 and 46 days in the forest floor extracts and oxic fen extracts, respectively. In comparison, DMA was stable for 100 days in anoxic fen extracts. The apparent half-lives were much shorter in soils for DMA (1.3–12.6 days) and AsB (0.5–1.9 days) than in soil extracts, suggesting also irreversible AsB and DMA adsorption to soils beside demethylation. An unknown arsenic species and DMA were detected as metabolites of AsB demethylation. The results indicate rapid demethylation of AsB probably via the pathway AsB → Dimethylarsenoylacetate → DMA, followed up by slow demethylation of DMA → monomethylarsonic acid → inorganic As species.