Interfacial tension of CH(4) + kinetic inhibitors, inhibex 301, and inhibex 501 systems was measured at different concentrations of hydrate inhibitors using the pendant-bubble method. The temperature and pressure ranges were (274.2 to 282.2) K and (0.1 to 20.1) MPa, respectively. The experimental data show that the interfacial tension between methane and aqueous solution of hydrate inhibitor decreases with the increase of pressure and inhibitor concentration. It implied that the interface behavior of inhibex 301 and inhibex 501 is similar to a surfactant. The presence of inhibex 301 in water makes the interfacial tension decrease more remarkably compared with inhibex 501.