This paper is concerned with the study of the effect of the molecular structure of surfactant on the reduction of interfacial tension of Enhanced Oil Recovery (EOR). The interfacial tension behavior of nonane and crude oil with three hexadecylbenzene sulfonate isomers (with benzene rings located at different positions along the alkyl chains) were investigated. It was found that the interfacial tensions could be reduced to ultra-low values at low alkali concentrations by using a surfactant molecule with a phenyl group located near the center of the alkyl chain. Molecular structures of this kind were considered to be the most ideal for surfactant flooding. Moreover, the transient low interfacial tensions in crude oil/alkali system were caused by the synergistic effects between surfactant and the active species generated at interface, but surfactant molecules played the dominant role at equilibrium. (c) 2005 Elsevier Ltd. All rights reserved.