Alloys of Fe-15Mn, Fe-25Mn and Fe-50Mn were reacted with CO/CO 2/SO 2/N 2 atmospheres at 1073K, and their behaviour compared with that of the individual metals. Under reducing and sulfidizing conditions, uncatalysed gases led to irreproducible results. Catalysed gases led to sulfide scale growth on manganese-containing alloys via reaction with COS, whereas the scale on pure iron was largely oxide formed by reaction with SO 2. A gas containing moderate sulfur and oxygen activities and a high partial pressure of SO 2 corroded the alloys by reaction with SO 2, forming scales of oxide on top of sulfide-plus-oxide mixtures. Very high manganese levels were required to produce any reduction in rate. A gas containing a low sulfur activity and a moderate oxygen activity led to reaction with SO 2, producing oxide-plus-sulfide scales which were in most cases of lamellar morphology, and fast-growing. The kinetics of lamellar product growth were initially linear and then parabolic, as the scale-gas interface approached a steady state. In all cases, the addition of manganese to the alloys promoted a more rapid approach to local equilibrium at the scale-gas interface.