A thermodynamically stable interface can only exist between two immiscible fluids, nonetheless in this work an "immiscible interface" between two gases (CO2-methane) has been observed and is documented within this paper This work is the first ever attempt to measure the interfacial tension (IFT) in a gas-gas system, the interfacial tension of supercritical carbon dioxide (SCO2-methane) was measured at temperatures of 368.15 and 433.15K and pressures between 69 and 41 37 MPa, using a modified pendant drop method This study was undertaken to investigate the effects of CO2 injection into a gas reservoir or enhanced gas recovery It was noted that the IFT decreased linearly with both temperature and pressure in the low-pressure range, but was less sensitive at higher pressures There was a zone in the vicinity of 10 34 MPa and above that was noted to be independent of temperature where IFT increased very sharply. The IFT was almost threefold higher at 20.69 MPa, for the same temperature, compared with 10 34 MPa This is attributed to the density of SCO2 at 10 34 MPa being less than 1/3 of density at 20 69 MPa, at the same temperature It was also evident that the SCO2-methane IFT was relatively lower at higher temperature; a behavior seen in density and viscosity of SCO2 The IFT results at 41 37 MPa and 433 15K were very close to the 20 69 MPa and 368 15 K The increase in temperature slightly influenced IFT and was matched by the pressure effect on IFT Hence the overall impact remained almost the same (C) 2010 Elsevier B V All rights reserved