This work examines the difference in behaviour between two austenitic steels, 304L and 310S, in a simulated environment with carbon and oxygen activities typical of the syngas cooler of a steam blown gasifier. The cyclic exposures were performed with and without addition of 500 ppm HCl for a total duration of 960 h. The oxide layers (200-600 nm) were studied using TEM, a focused ion beam microscope with EDX (FIB-EDX), electron back scattered diffraction (EBSD), SEM and X-ray diffraction (XRD). For 304L, a Cr and Ni depleted zone with a ferritic structure was observed below the oxide layer and a thin, manganese rich layer at the metal/oxide interface. For 310S, circular oxide patches were observed on the surface after exposure with HCl. These displayed elevated concentration of Al, Ca and Si, typical of inclusions. Results indicate that the addition of HCl results in a difference in oxide quality and structure but that HCl at the level of this study has little impact on the sound metal thickness.