The bio-geochemical processes of actinides potentially released to the environment from nuclear waste repositories are mainly determined by their chemical form. Results of direct and indirect determinations of soluble and colloidal species of radionuclides are reported. Experimental investigations on americium speciation in sea water are compared with model calculations. Americium pseudocolloids smaller than 0.45 m were identified. The application of the Specific Interaction Theory to activity coefficient correction is shown for the chloro- and sulphate complexes of americium. Analytical techniques for direct speciation studies are briefly reviewed.