Chemical process design is a creative step of engineers that should be supported by different computer-aided design tools. Such tools should be simple and easy to use because process synthesis often means the investigation of a huge number of alternatives. During the design of the separation of liquid mixtures, among many others, distillation and pervaporation are usually simultaneously considered because pervaporation is frequently considered as an alternative to distillation. To easily compare the efficiencies of continuous pervaporation and flash distillation, we propose a new and simple method, the so-called membrane flash index (MFLI). The comparison is based upon vapor liquid equilibrium data and the permeation data of a pervaporation membrane and therefore can be plotted on a common chart in the case of binary mixtures. The permeation values of organophilic and hydrophilic pervaporations can be calculated with the help of known separation factors and feed concentrations. The MFLI is dimensionless and can be determined by dividing the permeate concentration by the corresponding vapor equilibrium data obtained from a simple flash distillation. Only the feed concentration, separation factor, and refereed equilibrium data are necessary for the calculations. If the MFLI is <1, the application of pervaporation can be worse than that of flash distillation. In our study, the efficiencies of pervaporation and flash distillation are compared using three mixtures, methanol/water, ethanol/water, and isobutanol/water. After determining the MFLI for different types of membranes, we can conclude that the separation capability of hydrophilic membranes is significantly better than that of the organophilic ones and also better than that of flash distillation by far. Apart from azeotrope braking, the MFLI, as a single number, gives clear information about the separation capabilities of pervaporation and flash distillation in the course of process design.