Two sodium branched‐alkylbenzensulfonates with additional alkyl substituents were synthesized through a series of reactions. The interfacial tension of these alkylbenzenesulfonates between 1.0% NaCl solution and six n‐alkanes were measured. From the data of measurements the following values were calculated: critical micelle concentration (cmc), the interfacial tension at the cmc (γcmc), interfacial excess concentration at the cmc (Γm), area per molecule at the cmc (Am). There were a minimum γcmc and a maximum Γm appeared for the same n‐alkane with increasing the hydrocarbon chain length of the oil. These indicated that the hydrocarbon chain length of oil have the important effect on adsorption and interfacial tension.