An on-line flow injection method for the direct determination of silver in sea-water using isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) is described. A mini-column packed with 50 µl Dowex 1-X8 anion exchange resin was used to separate and concentrate Ag from sea-water following on-line merging with 0.5 M HCl. Recoveries of 100±1% and 96±1% were obtained for NRC CRMs SLEW-3 and CASS-4, respectively. One run can be completed in less than 6 min using a 12 ml volume of sea-water. The detection limit (3σ), estimated at 0.06 pg ml−1, is superior to previously published methods. Dissolved Ag concentrations of 1.93±0.10, 5.15±0.10 and 5.42±0.10 pg ml−1 were obtained in SLEW-3, CASS-2 and CASS-4 CRMs, respectively. The method was also assessed for determination of Ag in a marine sediment CRM MESS-3, and a concentration of 0.169±0.002 µg g−1 was obtained using ID ICP-MS after an off-line column separation, in agreement with the certified value of 0.18±0.02 µg g−1. The on-line method was also applied to analyses of sea-water and sufficiently diluted digests of marine sediments using an external calibration technique. Concentrations of 1.96±0.10 pg ml−1 (mean and one standard deviation, n = 3) and 5.55±0.15 (n = 3) pg ml−1 were obtained for dissolved Ag in SLEW-3 and CASS-4, respectively, in agreement with results generated by ID ICP-MS. A concentration of 0.181±0.004 (n = 3) µg g−1 in MESS-3 was obtained, based on 200-fold diluted digests, in agreement with the certified value.