The International Atomic Energy Agency (IAEA)'s Environmental Modelling for Radiation Safety (EMRAS) model evaluation programme includes a Tritium and Carbon-14 Working Group (TCWG), the goal of which is to test the performance of models for the environmental transfer. This paper describes work on two of the nine TCWG scenarios; one involved the prediction of time-dependent tritium concentrations in freshwater mussels subject to an abrupt change in ambient tritium levels and the second was concerned with the prediction of carbon-14 concentrations in rice grown in the vicinity of sources of continuous atmospheric releases.