Arsenopyrite has traditionally been considered to be chemically unstable in the surficial environment. However, field evidence demonstrates that arsenopyrite does not readily decompose under water-saturated near-surface conditions. Arsenopyrite occurs in the heavy mineral suite of a variety of thin sediments in southern New Zealand, with no evidence for chemical corrosion. These sediments have been water-saturated but within metres of the surface for 10, 60, 28000, and >2 million years. These observations of long-term arsenopyrite stability are in accord with geochemical predictions based on recent experimentally determined thermodynamic data for arsenopyrite. Calculations using the new thermodynamic data suggest that arsenopyrite has similar Eh–pH stability range to pyrite, except under acid conditions (pH<4) where arsenopyrite should transform to realgar or orpiment. Calculated As concentrations in equilibrium with the pyrite–arsenopyrite assemblage in the surficial environment are 0.01–0.1 ppm, similar to groundwater in arsenopyrite-bearing rocks in this study (0.005–0.3 ppm), and these waters may be in chemical equilibrium. Arsenopyrite decomposes in oxidised waters to yield up to 1100 ppm dissolved As in the laboratory and up to 400 ppm dissolved As in mine processing waters. These concentrations of dissolved As are several orders of magnitude lower than equilibrium solubility of arsenopyrite because of kinetic effects and development of protective oxide coatings on arsenopyrite grains. Mine tailings containing arsenopyrite should be chemically stable during long-term storage provided they are kept water-saturated and moderately reduced.