Plutonium (Pu) isotope ratios can be used to differentiate between sources of Pu contamination such as nuclear weapon production, weapon fallout as well as accidental and routine releases from nuclear installations. To obtain information on the contamination level, speciation and sources of Pu in the Ob and Yenisey river systems (Siberia, Russia) and the adjacent Kara Sea, water was size fractionated onboard ship and the concentrations and atom ratios of 240Pu and 239Pu in obtained water fractions (i.e. particles, colloids and low molecular mass species) were determined by accelerator mass spectrometry (AMS). Results show a clear difference in speciation between high 240Pu/239Pu atom ratio Pu derived from global weapon fallout and low 240Pu/239Pu atom ratio Pu, presumably originating from weapons grade Pu. In particular, the 240Pu/239Pu atom ratios (mean 0.18 ± 0.06) for particles (> 0.45 μm) could not be distinguished from global fallout Pu (0.17–0.19), whereas for low molecular mass (LMM
