[en] Sequential extractions are used to evaluate the mobility and bioavailability of natural radionuclides in the environment as well as their binding and origin in soil. However, as extractions results are dependent on the extraction conditions, it is not possible to find out the exact chemical forms of the radionuclides or their sorption to certain geochemical phases. In sequential extractions the same solid sample is extracted several times with stronger extraction conditions each time. There are many extraction methods but their fractionation is mostly same. Easily exchanged cations are extracted in the first step. In the second step acid-soluble materials are dissolved and in the third step reducible manganese and iron oxyhydroxides are dissolved. In the fourth step the sample is extracted with oxidizing reagent to decompose the oxidisable materials. In the last extraction step the remnant sample is decomposed with strong acids. This study focuses on sequential extractions of Olkiluoto soil samples. Five-step extractions were carried out to find out more information about the sorption of uranium, thorium, radium and lead in the soil. The concentrations of uranium, thorium and stable lead in extraction fractions were determined with inductively coupled plasma mass spectrometry. ²¹⁰Pb activity concentration was determined by alpha spectrometry through its daughter nuclide ²¹⁰Po. Fractionation of the radionuclides was found to be dependent on the sample depth and particle size. The largest concentrations of uranium, thorium and lead were found in the smallest particle size samples. (orig.)