A unified theory is proposed to explain the ultra-low inter-facial tension minimum observed in dilute petroleum sulfonate solution/ oil systems encountered in tertiary oil recovery process. The relative distribution of the surfactant in oil and aqueous phase was determined using UV spectrophotometry as well as the monolayer technique. The critical micelle concentration ( CMC) of the aqueous phase equilibrated with oil was measured using surface tension, osmotic pressure and light scattering measurements. For three major variables namely, the surfactant concentration, the chain length of oil and the salinity of brine, the interfacial tension minimum was found to occur whenever the equilibrated aqueous phase was at CMC. This minimum in interfacial tension also coincided with partition coefficient of unity for surfactant distribution in oil and brine. The increase of the interfacial tension after reaching a minimum was attributed to the decrease in the onomer concentration in the aqueous phase. Three possible causes of the decrease in monomer concentration are (1) he incorporation of oil soluble fraction into mixed micelles in the aqueous phase, or (2) the decrease of the effective CMC in the equilibrated aqueous phase due to solubilization of oil or (3) the transfer of surfactant from aqueous to oil phase upon increasing the salinity. The decrease in surface concentration of surfactant at the oil-brine interface after the interfacial tension minimum is reached was confirmed by the electrophoretic mobility measurements