Portable cavity ring-down spectrometers (e.g. G1101-i) for isotopic CO2 have an interference with elevated (higher than ambient) amounts of hydrogen sulfide (H2S). This results from the overlap of the H2S and CO2 spectral lines in the near-infrared, causing changes in both the 12CO2 and 13CO2 concentrations. This effect can be removed by reacting H2S with a metal scrub before analysis, which will facilitate the application of these instruments in H2S-rich environments (i.e. active volcanoes).