[en] Speciation studies refer to the distribution of species in a particular sample or matrix. These studies are necessary to improve the description, understanding and prediction of trace element kinetics and toxicity. In the case of internal contamination with radionuclides, speciation studies could help to improve both the biokinetic and dosimetric models for radionuclides. There are different methods to approach the speciation of radionuclides in a biological system, depending on the degree of accuracy needed and the level of uncertainties accepted. Among them, computer modelling and experimental determination are complementary approaches. This paper describes what is known about speciation of actinides in blood, GI tract, liver and skeleton and of their consequences in terms of internal dosimetry. The conclusion is that such studies provide very valuable data and should be targeted in the future on some specific tissues and biomolecules. (author)