The dilational properties of adsorbed film of sodium 2,5-dialkyl benzene sulfonates at the air-water and decane-water interfaces have been investigated by a Langmuir trough method. The influences of oscillating frequency and bulk concentration on dilational modulus and phase angle were expounded. The experimental results showed the hydrophobic chains located at different positions have different effects on dilational properties: for the air-water interface, the short alkyl chain ortho to sulfonate group produces a small increase in dilational modulus, while an increase in long para-alkyl plays an important role in the surface relaxation process, which leads to a higher surface dilational modulus. In the case of the decane-water interface, the interfacial dilational moduli are obviously lower than those of the air-water interface due to the insertion of decane molecules from oil phase. Moreover, the strong interactions between para-alkyl chains were weakened by decane molecules; therefore, the interfacial dilational moduli of sodium 2-ethyl-5-(1-butyl)octylbenzene sulfonate (4-7-2) and sodium 2-methyl-5-(1-hexyl)octylbenzene sulfonate (6-7-1) are approximately equal. The possible schematic diagrams of adsorbed molecules with different structures at the water-air and water-decane interfaces are proposed.