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Many decisions in environmental management are based on model predictions that are plagued by uncertainty. It is sensible to consider this uncertainty because it can influence the outcome of the decision-making process. This paper outlines a method to assess uncertainty in model predictions in situations in which validation research is difficult or impossible. It is illustrated in a case study of the coherenceof independently derived environmental quality objectives (EQOs) forair and water involving the application of a multimedia fate model (SimpleBox). Distinction was made between operational and fundamental uncertainty, which were assessed separately. Operational uncertainty was assessed by quantifying the uncertainties in the input parametersof the model and performing Monte Carlo simulations. Fundamental uncertainty was assessed tentatively by means of a postal questionnaire among scientists and scientifically trained policy makers and representatives of interest groups in the Netherlands. The results indicate that the perceived fundamental uncertainty tends to exceed the calculated operational uncertainty. These results emphasize the importance of considering fundamental uncertainty in environmental decision making. Efforts to improve the quality of coherence testing of independently derived EQOs should aim at reducing the perceived fundamental uncertainty, for example, by defining the coherence criteria of EQOs in measurable attributes, thus facilitating validation research and enabling a more detailed assessment of the fundamental uncertainty. [ABSTRACT FROM AUTHOR]