Abstract <p>The results of long-term studies on the main forms of migration of manmade radionuclides (⁹⁰Sr, ¹³⁷Cs, ^239,240Pu) in river and marine ecosystems, performed at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, are presented. The data on the relative content of radioisotopes in various size fractions (in suspension, particle size larger than 0.45 μm; in solution, particle size smaller than 15 kDa; and in colloids with intermediate-size particles) reveal the influence of the hydrochemical regime of freshwater and marine ecosystems on the redistribution of radionuclides with respect to the size of particles accumulating them. The transport of radionuclides in the river system can occur in several forms. Cesium-137 migrates mainly in a form firmly fixed on the suspended matter and, to a lesser extent, in a soluble form. Strontium-90, on the contrary, migrates mainly in a soluble form and partly in the form of colloids of various sizes. Plutonium-239,240 is associated with large colloidal particles and suspended matter. In marine ecosystems, ¹³⁷Cs, on the contrary, it is mainly in the dissolved state, and ⁹⁰Sr is almost equally in the colloidal and dissolved fractions. Plutonium-239,240 is significantly bound to the suspended matter and is also equally distributed between the colloids and the dissolved fraction.</p>