A micellar slug for use in the recovery of oil, consisting essentially of a surfactant and an aqueous medium, which may contain an inorganic salt. The surfactant contains:(a) at least one alkoxy sulfonate having the general formula (I): ##STR1## wherein R.sup.1 represents an alkyl group having 1 to 26 carbon atoms, R.sup.2 represents an alkyl or alkenyl group having 1 to 24 carbon atoms, R.sup.3 represents hydrogen or an alkyl group having 1 to 24 carbon atoms, n is a number of 0 to 5, and X represents an alkali metal, an alkaline earth metal, ammonium, or an organic amine, and(b) at least one other surfactant.The micellar slug obtains large intake amounts of oil and brine in the resultant micro-emulsions and a good dilution stability of the micro-emulsion, and is capable of recovering oil from subterranean reservoirs having a high salt content at a high oil recovery efficiency.