A diffusive gradients in thin films (DGT) technique for selectively measuring AsIII utilizes commercially available 3-mercaptopropyl- functionalized silica gel. Deployment of the new technique alongside the Metsorb-DGT for total inorganic arsenic allows the calculation of As III directly and AsV by difference. Uptake of As III by mercapto-silica was quantitative and elution with a mixture of 1 mol L-1 HNO3 and 0.01 mol L-1 KIO 3 gave a recovery of 85.6 + 1.7%. DGT validation experiments showed linear accumulation of AsIII over time (R2 > 0.998). Accumulation was unaffected by varying ionic strength (0.0001-0.75 mol L-1 NaNO3) and pH (3.5-8.5). Deployment of mercapto-silica DGT and Metsorb DGT in seawater spiked with AsIII and AsV demonstrated the ability of the combined approach to accurately quantify both species in the presence of potential competing ions. Ferrihydrite DGT, which has been previously reported for the measurement of total inorganic arsenic, was evaluated in seawater and shown to underestimate both AsIII and AsV at longer deployment times (72 h). Reproducibility of the new mercapto-silica DGT technique was good (relative standard deviations < 9%), and the average method detection limit was sufficiently low to allow quantification of ultratrace concentrations of AsIII (0.03 ¿g L-1; 72 h deployment). 2011 American Chemical Society.