A study to perform an uncertainty analysis of the European accident consequence assessment system, Cosyma, has been carried out under contract to the European Commission. The study involved a series of analysis of the uncertainty in different sections of the system, followed by a final analysis of the uncertainty in the whole system. The overall aims of the study can be summarised as: - to formulate a state-of-the-art expert judgment methodology which is capable of finding broad acceptance; - to apply the methodology to estimate uncertainties associated with the predictions of the probabilistic accident consequence system, Cosyma; - to provide an input to identifying future research and development priorities. Uncertainties analysis involves specifying probability distributions for the values of each of the parameters involved, sampling sets of values from those distributions and propagating them through the model to derive information on the uncertainty in the model prediction. Those parameters whose uncertainties make major contributions to the overall uncertainty can then be identified using correlation coefficients between the input values and the model outputs.