In this study, multiple types of samples, including soils, plants, litter and soil invertebrates, were collected from a former arsenic (As) mine in China. The total As concentrations in the soils, earthworms, litter and the aboveground portions of grass from the contaminated area followed the decreasing order of 83–2224 mg kg−1, 31–430 mg kg−1, 1–62 mg kg−1 and 2–23 mg kg−1, respectively. XANES analysis revealed that the predominant form of As in the soils was arsenate (As(V)). In the grass and litter of the native plant community, inorganic As species were the main species, while minor amounts of DMA, MMA were also detected by HPLC-ICP-MS. The major As species extracted from earthworms were inorganic and AsB was the only organic species present in the earthworm samples, although at low proportions. The internal bioconversion of other As species is hypothesized to contribute greatly to the formation and accumulation of AsB in earthworms, although the direct external absorption of organic As from soils might be another source. This study sheds light on the potential sources of complex organoarsenicals, such as AsB, in terrestrial organisms.