During the petroleum dehydration process, it is necessary to use chemical demulsifiers to break the water-oil emulsions that are formed during oil extraction. The majority of the products used are formulations containing nonionic surfactants based on poly(ethylene oxidepropylene oxide) (PEO-PPO) block copolymers with different ethylene oxide/propylene oxide molar ratios. In this work, PEO-PPO block copolymers of different structures were used. The physical-chemical properties of aqueous PEO-PPO solutions and their mixtures were analyzed, along with their effectiveness as water-oil emulsion destabilizing agents. The results showed that all the PEO-PPO copolymers could reduce the interfacial tension between water and oil. Nevertheless, the most efficient water-oil emulsion demulsification was achieved by the PEO-PPO branched copolymer, whose structure presented hydrophilic segments [poly(ethylene oxide) and OH] side by side at the free end of the molecule. This copolymer also exhibited the highest water solubility. Such behavior could be attributed to its structure, which promoted better interaction with the water droplets dispersed in the water-oil emulsion. The performance of the surfactant mixtures appeared to be related to their capacity to reduce the water-oil interfacial tension. The addition of a surfactant at a concentration of roughly 30% without demulsifying action does not compromise the action of a well-performing surfactant. (c) 2007 Wiley Periodicals, Inc.