Arsenic (As) is a toxic metalloid that is present ubiquitously in the environment. Due to mining and smelting, arsenic contamination is widespread in some paddy fields in southern China. Also, in these areas human are exposed to As mainly through rice. When paddy soils are flooded, the mobility of As and the capacity of As methylation are enhanced casuing dimethylarsenate (DMAs) to accumulate in the soil solution. However, DMAs can also be demethylated or volatilized by microbes. Demethylation and volatilization of DMAs from paddy soils were investigated in the present study. First, we incubated three paddy soils collected from different areas in China under flooded condition. During incubation, DMAs was produced in first two weeks and then disappeared. The addition of 2-bromoethanesulfonate (BES), a specific inhibitor of methanogens, significantly decreased the abundance of methanogens and retarded the disappearence of DMAs. When exogenous DMAs was added to the three paddy soils, DMAs was demethylated into monomethylarsenic (MMAs) and inorganic As, which was inhibited by BES. Methanogens enrichment cultures were established from the three soils under anaerobic conditions. The enrichment cultures were able transform DMAs to As(III) and monomethylarsenate (MMAs). We synthesized ¹³C-DMAs using two enzymes in vitro, and added ¹³C-DMAs into the paddy soils. After incubated for 14 and 40 days, ¹³CH₄ was detected in the headspace. These results indicate that methanogens play a major part in arsenic demethylation from DMAs. Secondly, we determined As volatilization from DMAs(V) added to two paddy soils. The addition of molybdate, a specific inhibitor of sulfate reducing bacteria (SRB), greatly enhanced dimethylarsine (DMAs(III)H) evolution from DMAs(V). Molybdate addition significantly shifted the microbial community structure in two paddy soils. Aslo, a strain Bacillus sp. CZ-2 capable of ruducing DMAs(V) to DMAs(III)H was isolted from the soil used, and its volatilization ability was enhanced by Mo. This bacterial strain could not methylate As(III) or MAs(V). RNA-seq analysis identified 10 reductase genes upregulated by Mo in Bacillus sp. CZ-2, which may be involved in the reduction of DMAs(V) to DMAs(III)H. Our results show that DMAs(III)H evolution from DMAs(V) is independent of the As methylation pathway, and that Mo enhances DMAs(III)H evolution from paddy soils by shifting the microbial community structure and enhancing the reduction of DMAs(V) to DMAs(III)H, possibly through upregulating the expression of DMAs(V) reductase gene(s).