This paper reports the first online gas phase detection of absolute concentrations of HONO under engine relevant conditions, during the oxidation of an alkane in the presence of NOx. The detection was achieved at laboratory scale thanks to the coupling of a jet-stirred reactor to a continuous-wave Cavity Ring-Down Spectroscopy cell. The evidence of the formation of HONO was obtained by comparing measured cw-CRDS spectra with a literature one. The formation of HONO was simultaneously observed with the appearance of another nitrogen compound: NO2. This confirms that HONO could also be formed from NO2 under engine conditions.