The first reliable quantification of hydrogen peroxide (H2O2) formed during the low-temperature oxidation of an organic compound has been achieved thanks to a new system that couples a jet stirred reactor to a detection by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared. The quantification of this key compound for hydrocarbon low-temperature oxidation regime has been obtained under conditions dose to those actually observed before the autoignition. The studied hydrocarbon was n-butane, the smallest alkane which has an oxidation behavior close to that of the species present in gasoline and diesel fuels.