Data uncertainties are present in any engineering calculation. Their impact may be particularly important if they concern safety-related parameters of process plants. Procedures are presented to deal with such uncertainties. Their propagation through calculations by means of a Monte-Carlo approach is shown. The adiabatic temperature rise and the time for achieving a specified fraction of conversion are evaluated for an example, the esterification of acetic anhydride, in order to show the influence of uncertainties and to demonstrate that the conventional procedure of using point values may produce a misleading basis for design decisions.