In process synthesis, cost-effective process flowsheets must be generated and unpromising flowsheet alternatives must be discarded. Currently, there are no simple economic criteria for comparing alternatives to distillation columns for vapour-liquid equilibrium (VLE) separation processes. This work introduces a pair of ’capacity variables’ which may serve as cost indicators for processes involving vaporization and condensation. These variables are defined for an equilibrium stage and extended to describe a separation process comprised of many such stages. These definitions are applied to a range of VLE separation processes, and consistent results are obtained for all the processes considered. The capacity variables for the process are found to be related to the total internal vapour and liquid production requirements, which are related to both capital and energy costs of the process. The overall capacity variables are calculated for a simple ideal binary distillation column. The capacity variables are seen to follow closely trends in cost. The optimum redux ratio predicted by the capacity variables is around 1.5 times the minimum reflux ratio, which is close to the accepted heuristic rule of 1.03-1.3 times the minimum reflux ratio. The capacity variables are thus shown to be potentially useful variables for assessing costs in process synthesis exercises.