The behaviour of high alloy steels, especially ferritic, A1-containing alloys based upon Fecralloy®, has been studied in simulated coal gasification atmospheres (CGAs) containing HC1 at 450°C. In addition, alloy modifications with increased Si and Cr levels were studied. Samples were also exposed in gasification pilot plants to compare with the behaviour observed in the laboratory. In CGA tests containing 1000 ppm HC1, which simulated conditions in an atmospheric gasifier, all the Fecralloy-based alloys tested exhibited low rates of attack, so these materials would be suitable for long-term exposure in these conditions. It was shown that, while initial formation of sulfides occurred, an Al-rich layer was eventually established that was rate controlling and prevented further formation of sulfides. Prior oxidation before exposure to the CGA prevented sulfide formation, and the scale composition and morphology that were observed in this case correlated well with those observed in the pilot plant. At levels of HC1 that would correspond to the operation of a pressurized gasifier, greatly increased rates of attack were observed and a thick sulfide layer was developed which, during the early stages of the reaction, was disrupted by chloride formation. The mass change data indicated that rates of attack for all the alloys examined were considered borderline for this application.