Accelerator mass spectrometry (AMS) has been used to measure Pu activities and 240Pu/239Pu isotope ratios in samples contaminated by releases from the Mayak nuclear installation. Determination of Pu isotopes in high-level samples indicated that the ratio of 240Pu/239Pu in waste has increased toward the present. The lowest 240Pu/239Pu atom ratios, 0.012−0.024, were found at the Asanov Swamp, where the primary source of contamination was discharge of intermediate-level radioactive waste between 1949 and 1951. The highest ratios, 0.06−0.29, were found in industrial reservoirs contaminated by various sources of waste up to the present day. Measurement of Pu isotopes in low-level samples collected from the Techa, Iset, and Ob Rivers showed that while activity levels decrease with distance from Mayakfrom 2000 Bq/kg at 7 km downstream to less than 1 Bq/kg sediment at 250 km240Pu/239Pu isotope ratios increase. Results suggest that most of the plutonium in the Upper Techa River originates from the early waste discharges, although enhanced atom ratios in surface sediments downstream (0.035−0.099) indicate a contribution from other sources. On the basis of procedural blanks, detection limits for AMS were below 1 fg of Pu.