AbstractRising energy costs have renewed interest in energy efficient separations. Such efficiencies can be defined as the free energy of unmixing divided by the work done on and the heat added to the process. The greatest efficiencies occur when the energy cost far exceeds any capital cost. In this limit, rate processes are not important, and efficiency is governed by thermodynamics. Efficiencies for gas absorption and stripping are then typically around 50%. The efficiencies of liquid–liquid extractions are lower, especially when one solute is wanted at higher concentration. The efficiencies of membrane separations are higher, but these require high pressure, normally obtained mechanically. As a result, membrane efficiencies should be reduced by any Carnot efficiency of generating the mechanical energy. These results, which are consistent with earlier estimates for distillation, have implications for carbon dioxide capture as a route to mitigating global warming. © 2012 American Institute of Chemical Engineers AIChE J, 2012