The single crystal MD2 (Ni-8Cr-5Al-5Co-2Mo-8W-6Ta-1Ti) and the polycrystalline Inconel 792 (Ni-13Cr-3Al-9Co-2Mo-4W-4Ta-4Ti) were exposed at 850 and 900°C in an severely sulfidising environment comprising synthetic air with 8% water vapour and 2000 ppm SO2, and a deposit of 20mol% Na2SO4 + 80mol% K2SO4 applied every 160 hours. Results were evaluated in terms of mass change, which indicated the existence of an incubation time before the onset of extensive attack, and post-test metallography to evaluate metal loss. Metallographic studies are presented showing the complex external oxide scale formation and internal oxidation and sulfidation. © 2009 Science Reviews 2000 Ltd.