A new method was used for experimental simulation of the sorption–desorption transformation of microelement composition of the adsorbed complex of solid substances of river runoff at the interaction with seawater. The experimental results suggest the conclusion that a change of freshwater to marine conditions is accompanied by desorption of some elements and sorption of others. The desorption increases the runoff into the World Ocean of dissolved forms of manganese, cobalt, nickel, cadmium, thallium, and barium by 102, 21.2, 4.0, 16.2, 4.8, and 21.1%, respectively, while the sorption leads to a decrease of the global river runoff of dissolved lead and cesium by 9.4 and 2.6%.