This paper reports on an experimental investigation* of the concentration field of supercritical jets of methane and hydrogen. The jets were produced by venting a tank containing gas at a pressure of 40 bar (568 psi) to atmosphere through orifices whose diameters ranged from 25 to 150 mm. The concentration measurements were made in the subsonic zone of the jets using an original technique. This technique employs a sensor using the catalytic oxidation effect and can measure the concentration of combustible gas in highly reactive environments with no risk of ignition. The investigation was designed with experiments on a scale typical of that encountered in industry during accidental discharges of pressurized gas, a scale much larger than that normally used in a laboratory. The results show that when proper allowance is made for the expansion zone of supercritical jets it is possible to relate the results obtained in these jets with those found in variable density subsonic jets. In particular, the rates at which mass concentration decreases along the axes of supercritical and subsonic jets are equal when weighted by an equivalent diameter which takes into account the conditions of both pressure and density in the discharge section.