Abstract.lt is well known that during the years after a nuclear accident the bioavailability and environmental mobility of radionuclides may change significantly, resulting in significant changes in contamination of foodstuffs and surface waters. Studies on l37 Cs and, to a lesser extent, Sr, have quantified these changes in some ecosystems. However, variability in temporal changes of these radionuclides in aquatic and terrestrial systems is not yet well quantified. Estimation of such variability is a key component of any predictive model for long-term transfers of radionuclides in the environment. We have analysed measurements (from both weapons testing and Chernobyl deposits) of 13, Cs in runoff waters and in aquatic and terrestrial foodstuffs in order to determine temporal changes in their bioavailability and mobility. Using these empirical data, and the results of parallel modelling studies of vertical migration of radiocaesium in soils, we have quantified the relative importance of transport processes of l37 Cs, as compared to slow changes in its chemical availability in the soil. On the basis of these results, we have developed simple models for predicting time changes in activity concentrations of l37 Cs in surface water, foodstuffs and the human body during the years to decades after radioactive fallout. Importantly, we also determine uncertainties in model parameter estimates, and highlight the potential causes of this uncertainty.