The beneficial effect of a series of nonylphenol polyethylene glycol ether surfactants were tested in the flotation of phosphate ores in which the mixtures of sodium oleate and dodecane were used as collectors. In order to elucidate the flotation results, equilibrium surface tension of single and binary surfactant solutions prepared with sodium oleate and nonionic surfactants were investigated. Synergism between surfactants was determined by observing the change in surface tension of solutions and by the calculation of interaction parameter from surface tension data. The lowering of surface tension of sodium oleate solutions was observed in the presence of nonionic surfactants. The interaction parameter between sodium oleate and a nonionic surfactant (i.e., NP-4) was calculated to be -0.21, showing attraction between the two surfactant molecules. The nonionic surfactant NP-4 was then tested in batch flotation of phosphate ores. The beneficial effect of nonionic surfactant was obtained at different collector dosages and surfactant combinations. (C) 2003 Elsevier Science Ltd. All rights reserved.