A systematic investigation of alcohol ethoxylates (AEOs) and their use as surfactants in the CO2 + water system was performed by measuring the interfacial tension (IFT) at 283 K and (5 to 20) MPa. It is the first time that the polydispersity of the AEOs has been studied. The synergistic effect was examined thoroughly. The findings enable the surfactant behavior to be expressed as the hydrophilic-CO2-philic balance, analogous to the HLB of the tradinional o/w systems, and also permit a relationship to be established between surfactant structure and the behavior at the interface.