This paper reviews the basic mechanisms governing tritium release rates from a spill of tritiated water and develops a mathematical model which can be used in these types of calculations. The model accounts for the mechanisms of evaporation and condensation at the water surface and water species exchange, and is applicable under a broad range of wind, temperature and humidity conditions. The model predictions are compared with a broad range of published data and other models, and the predictions are in good agreement with published experimental data.