Arsenic (As), a common contaminant in mining waste water, can be efficiently sequestered in peatlands used for waste water purification. However, alkaline, high sulfate waste water can modify peatland chemistry to conditions previously shown to favor As desorption through formation of highly mobile aqueous As-S species, so-called thioarsenates. To assess the risk of As re-mobilization, a peatland used for purification of mining waste water from a gold mine in Finnish Lapland was characterized in regard to peat and pore water As, sulfur (S) and iron chemistry. Additionally, an incubation experiment with two peat cores was conducted to investigate the influence of different S species on potential re-mobilization of As from peat and As speciation in pore water. High As retention in near-surface peat layers close to the waste water inflow could be confirmed. However, highest aqueous As concentrations occurred in 30-60 cm depth of the most contaminated, slightly alkaline area simultaneously with a peak of dissolved reduced S. At this peak, arsenite dominated speciation and up to 9% inorganic thioarsenates and 15% methylthiolated As species were found. The incubation experiments confirmed speciation patterns observed in the field and revealed high potential for As mobilization through thioarsenate formation (up to 98%) under neutral to alkaline, sulfate reducing conditions. The results of field assessment and incubation study were used to estimate potential risks of As mobilization. Highest potential As mobilization can be anticipated for stagnant conditions in near-surface peat layers upon transition from oxic to anoxic conditions.