Flow-leaching studies were conducted using borosilicate glass beads containing 241-Am, 237-Np and 238-Pu. The leachates were a bicarbonate groundwater and a brine. Tests were carried out unger oxidizing and reducing conditions. In bicarbonate groundwater (2.6 × 10−3 M NaHCO3, pH 8.4) under oxic conditions, steady-state leach-rates were found to increase in the order Pu∼Am<Np, with Np species leached congruently with sodium. Direct identification of Np carbonate complexes in the leachate was carried out by. electromigration. A reversed order of leachability was obtained in a reducing environment. In this case, Pu was found to be associated with particulate matter. In a salt brine leachate (5.4 M NaCl, pH∼6) a periodic cycle of increasing-decreasing concentrations was observed under reducing conditions. This was found to be dependent upon the redox state of actinides. Radionuclide adsorption into an unstable gel layer was postulated.