Arsenic speciation in environmental samples is important because toxic effects and mobility are determined by the chemical forms of the element present. The quantification of different As species is essential for studying toxicity and bio-transformation of As in aquatic and terrestrial environments. Although the inorganic species As(III) and As(V) have been considered dominant in soils and sediments, organisms are able to metabolize inorganic forms of arsenic into organo-arsenic compounds. This is most common in marine environments, where algae convert arsenate into arsenic-containing carbohydrates (arsenosugars). Also, algae are believed to use methylation as a detoxification mechanism. Arsenosugars and methylated compounds can be also found in terrestrial organisms, but they generally occur as minor constituents only. We investigated the dynamics of arsenic species under anaerobic conditions in soils and sediments surrounding gold mining areas from Minas Gerais State - Brazil, to elucidate the arsenic biogeochemical cycle and water contamination mechanisms. Soil suspensions were incubated under anaerobic conditions and periodically analyzed for soluble As species by IC-ICPMS and HPLC-ICPMS. Results showed that easily exchangeable As is mainly arsenite, except when reducible manganese is present, while arsenate is mainly responsible for the increase in soluble arsenic due to the reductive dissolution of either iron or manganese. The occurrence of organo-arsenic species was also found to be significant to the dynamics of soluble arsenic, mainly in soils from the Banded Iron Formation (BIF), under our experimental conditions. This unexpected occurrence is attributed to enrollment of ternary organic complexes or methylation by living organisms such as algae. In fact, we have successfully isolated some photosynthesizing microorganisms, such as Cyanophyta and Chlorophyta, from original soil samples after the experimental period.