Here we present the results of experiments concerning the adsorption of Thorium (Th) and Protactinium (Pa) on different particle types. Particle types used were smectite (as a representative of clay), biogenic opal, CaCO3 and MnO2. Additionally one sample was run without particles added. Experiments were performed with recently sampled seawater from three regionally different locations, which had been 0.2 µm filtrated previous to our experiments. Herewith we made sure that the seawater kept its natural composition in dissolved organic matter (DOM), which has been shown to play an important role in the transfer of Th to the particulate phase.In the control run with no particles added, in all three cases a significant amount of Th was observed in the particulate phase. Obviously, spontaneously formed particles as described by Chin et al. (1998) were responsible for Th found in the particulate phase. The adsorption of Th on clay strongly resembled that of the spontaneously formed particles, but with increased adsorption efficiency. In agreement with observations from terrestrial systems, we conclude that clay minerals may serve as a carrier for organic carbon also in the ocean. The relationship between so-called mineral ballast and carbon export (Armstrong and Jahnke 2001) may be closely linked to this observation.The obtained Th/Pa fractionation factors are only for smectite and MnO2 quite constant. For smectite, in all three water types a fractionation factor of about 6 is obtained. For MnO2, the fractionation factor scatters around unity. On biogenic opal, mostly fractionation factors of about two to three were observed.Our results have large implications for the interpretation of Th and Pa data from natural samples. They show up differences in the scavenging behaviour of particles depending on the composition of seawater in the phase <0.2 µm, which varies with its provenance.