Abstract: Separation of the uranium from Th, Pa, Fe and other elements by extraction chroma-tography is studied. A method is proposed for measuring uranium content and 234U/238U isotope ratio in natural water by Isotope Dilution Alpha Spectrometry (IDAS). Uranium is coprecipitated with Fe(OH)3 by NH4OH. The precipitate is dissolved in 2N HNO3 and the solution is passed through a column packed with CL-5209 Levextrel resin. Rinse the column with 1N HNO3 solution to strip iron, 4N HC1 solution to extract thorium, 4N HC1 +0.06N HF solution to extract protactinium. Uranium is then eluted by 0.3M (NH4)2C2O4 solution. After electroplating on a nickel disc, the uranium is determined by α-spectrometry. The precision of the determinations is≥±5 % for ppb level of U.