Optimal thermodynamic feed conditions for distillation of ideal binary mixtures are identified through thermodynamic efficiency analysis of distillation columns. Due to simplifying assumptions, temperatures do not appear explicitly in the final thermodynamic efficiency equations. These equations are found to be functions of only feed composition, the thermodynamic state of the feed and the relative volatility of the more volatile component A with respect to component B! When a single one-phase feed AB, is used the preferred state of the feed is all liquid when the amount of heavy component B is greater than 50% and is all vapor when the amount of volatile component A is greater than 50%. For a single two-phase feed a simple heuristic is presented to adjust the liquid fraction of a given feed to maximize the thermodynamic efficiency of the distillation column. For most binary mixtures, two feeds with the same composition, but one saturated vapor and the other saturated liquid can improve a distillation column’s efficiency substantially. The maximum benefit is obtained for a 50-50 mixture. This two-feed case is less efficient than the single two-phase feed case when the relative volatilities exceed certain calculable values.