Low energy separations for <1% CO2 gases would benefit gas treatment and CO2 sequestration. Theoretically, electrochemical pumping can separate and concentrate CO2 from the atmosphere or other gases with <1% CO2 at significantly lower energy cost than current systems. Principles of electrochemical pumping for CO2 separations are discussed and results for both organic solvent and ionic liquid working fluid systems are presented. Due to the large quantities of gases requiring processing during the separation/concentration of <1% CO2 gases, this work looked at solvents with negligible vapor pressures, specifically propylene carbonate and the room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. Other important parameters, as illustrated by the data and models presented, are low CO2 solubility in the solvent, high CO2 carrier solubility, CO2 binding constants, and the CO2 carrier's electrochemistry. Reported is the electrochemical pumping of CO2 from 0.5% (in nitrogen) to 100%, a 200-fold increase in partial pressure, using the CO2 carrier 2,6-di-tert-butyl-1,4-benzoquinone in a propylene carbonate solution. The ratio of CO2 moles pumped per electron mole was 0.43. The models determined the optimal CO2 solubility in the solvent and the required redox swing in the CO2 binding constants of the carrier. (C) 2003 The Electrochemical Society.