Interfacial tension data of (CH4 + N2) + H2O and (CO2 + N2) + H2O systems were measured using a pendant-drop method. The temperature and pressure ranges were 298-373 K and 1-30 MPa, respectively. The effects of gas composition, temperature, and pressure on the interfacial tension of the systems studied have been investigated. The linear gradient theory (LGT) was used to calculate the interfacial tensions of these two systems. The results show good agreement between the calculated and experimental interfacial tension data for the (CH4 + N2) + H2O system. However, it was found that the LGT model with one adjustable parameter is inadequate to give accurate predictions for the CO2 + H2O system and thus is not suitable for the description of the (CO2 + N2) + H2O system.