A soybean oil-in-water emulsion was prepared using nonionic Tween series surfactants. The effects of temperature, hydrophilic-lipophilic balance (HLB) value of the surfactant, and surfactant-to-oil ratio on the size of emulsion drops were investigated with an acoustic and electroacoustic devices. In the case of Tween 85, the influence of pH and electrolyte on the potential of emulsion drops was examined. zeta potential appears to be strongly dependent on pH, varying in the case of high concentrations of NaCl from +60 to -90 mV. High concentrations of Na+ and K+ are capable of separating the oil phase from the emulsion. Divalent cations such as Ca2+ and Mg2+ lead to two points of zero charge at high concentrations, which is not observed for the case of hydrolyzable trivalent species like Al3+ and Fe3+. (C) 2003 Elsevier Science (USA). All rights reserved.