In modem design simulators, the initial input for the estimation of the phase behaviour of mixtures is a series of characteristic parameters of the pure species, such as the normal boiling point T-b, the critical temperature T-c and the critical pressure P-c. For many components encountered in practice, these parameters are not available from experiment, and therefore, they must be estimated. The study of the propagation of errors in these estimated parameters through to the final design of an item of process plant - a distillation column separating binary and ternary mixtures - is the purpose of the present work. The likely magnitude of uncertainties in the input T-b, T-c and P-c is generated by the application of several popular and accepted methods. Considerable uncertainties in the predicted designs are demonstrated. Greater attention should be devoted to the development and employment in modem simulators of more sophisticated methods ensuring better description of molecular structures and intermolecular interactions.