A new diffusive gradients in a thin film (DGT) technique, using a titanium dioxide based adsorbent (Metsorb), has been developed and evaluated for the determination of dissolved inorganic arsenic and selenium. AsIII, AsV, and SeIV were found to be quantitatively accumulated by the adsorbent (uptake efficiencies of 96.5-100%) and eluted in 1 M NaOH (elution efficiencies of 81.2%, 75.2%, and 88.7%). SeVI was not quantitatively accumulated by the adsorbent (<20%). Laboratory DGT validation experiments gave linear mass uptake over time (R2 = 0.998) for AsIII, AsV, and SeIV. Consistent uptake occurred over pH (3.5-8.5) and ionic strength (0.0001-0.75 mol L-1 NaNO3) ranges typical of natural waters, including seawater. Field deployments of DGT probes with various diffusive layer thicknesses confirmed the use of the technique in situ, allowing calculation of the diffusive boundary layers and an accurate measurement of inorganic arsenic. Reproducibility of the technique in field deployments was good (relative standard deviation <8%). Limits of detection (4 day deployments) were 0.01 姠L-1 for inorganic arsenic and 0.05 姠L-1 for SeIV. The results of this study confirmed that DGT with Metsorb was a reliable and robust method for the measurement of inorganic arsenic and the selective measurement of SeIV within useful limits of accuracy.