ABSTRACT Surfactant-oil-water emulsified systems are found to exhibit a sharp change in viscosity near the optimum formulation for three-phase behavior. When the formulation is scanned through the three-phase transition by changing (in turn) brine salinity, oil EACN, surfactant HLB or alcohol concentration, the viscosity passes through a minimum for MOW three-phase emulsions. Bidimensional formulation scans corroborate that the viscosity minimum is associated with the optimum formulation. This minimum is related with the ultra-low interfacial tension exhibited by such systems.