Abstract: Samples of oil from a formation to be flooded are equilibrated with a series of surfactant systems comprising a petroleum sulfonate surfactant, a cosurfactant having limited solubility in water and brine. By using sulfonates having an equivalent weight within the range of 375 to 500 and using cosurfactants having a solubility in water varying from 0.5 to 20 grams per 100 grams of water, some of the mixtures will partition into two or more phases while others will not. Then a separate sulfonate-cosurfactant-brine system is made up corresponding to one of those which partitions and the system is injected into the reservoir where on contact with the oil it forms a multiphase bank.