An important safety function in ensuring nuclear waste safety is the possible formation of solubility limiting phases that incorporate actinides and other critical nuclides. For a credible demonstration of such a safety function, it is essential that uncertainties related to the choice of models, calculation cases, and parameter values are evaluated. In this paper, a method is outlined to evaluate thermodynamic data uncertainty and to analyse situations in the presence of uncertainty. Americium(III) solubility in an Aspo groundwater is taken as an illustrative example. Methods include a sensitivity analysis to single out critical parameters and a Monte Carlo analysis to provide an empirical probability distribution of americium solubility taking into account relevant uncertainty contributions. The analysis shows that the solubility products of Am-241(III) phases are the most sensitive parameters. Aqueous Am-241(III) species, on the contrary, are of less importance while the stability of several other complexes incorporating major element ions have considerable impact. (C) 2003 Elsevier Science Ltd. All rights reserved.