Multispectral vegetation reflectance measurements were used as an indirect method of sensing CO2 gas leaking from underground in a controlled release experiment in Bozeman, Montana, USA. The leak location is identified through time-series analysis of the reflectances and the normalized difference vegetation index (NDVI), evaluated at a test location and a control location. Vegetation reflectance changes that correlated with root-level CO2 exposure were distinguishable from changes attributed to seasonal factors including precipitation, wind, air temperature variation, etc. The NDVI of the vegetation became steadily smaller until saturating approximately twenty days after the beginning of the release. However, before reaching the threshold values, both reflectance and NDVI values changed more rapidly when exposed to elevated CO2 fluxes.