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Asymmetric flow-field flow fractionation-inductively-coupled plasma-mass spectrometry was used to determine whether colloidal arsenic (As) exists in soil pore water and soil extract samples at two arsenic-contaminated abandoned gold mines (Montague and Goldenville,Nova Scotia).Colloidal arsenic was found in 12 out of the 80 collected samples (=15％),and was primarily associated with iron (Fe) in the encountered colloids.The molar Fe/As ratios indicate that the colloids in some samples appeared to be discrete ironarsenic minerals,whereas in other samples,they were more consistent with As-rich iron (oxy)hydroxides.Up to three discrete size fractions of colloidal As were encountered in the samples,with mean colloid diameters between 6 and 14 nm.The pore water samples only contained one size fraction of As-bearing colloids (around 6 nm diameter),while larger As-bearing colloids were only encountered in soil extracts.