The effect of sediment size, pH, temperature and conductivity on the transfer of uranium from sediment to water has been studied. The uranium concentration and the234U/238U,235U/238U activity ratios were measured in water, sediments and suspended matter sampled from Jucar River, using low level alpha-spectrometry. Distribution factors were obtained from these measurements. A more detailed sampling was done in the neighbourhood of the Cofrentes Nuclear Plant (Valencia, Spain). Total uranium activity,234U/238U activity ratio and distribution factors for234U and238U were found to vary with pH. Leaching and dilution, which depend on pH and salinity, are the probable mechanisms for these changes.