The metal dusting behavior of Type 304L and 347 stainless steels (SSs) in a flowing mixed CO/H-2/H2O gas stream at 600 degrees C was investigated. After exposure in a 35% CO + 60% H-2 + 5% H2O gas for 500 h, large pits were formed on both steel surfaces. The aspect ratio of the pits formed in 304L SS was higher than that of the pits formed in 347 SS. The microstructures and chemical compositions of the reaction products and those of the substrate under the pits were examined by using a scanning electron microscope (SEM) combined with an energy dispersive spectrometer (EDS). A Cr-depleted zone containing voids was formed on the outer surface just beneath the pit. Massive matrix carburization and intergranular carbide precipitation were seen for both steels. The experimental results showed that niobium (Nb) could delay the ingress of carbon and retard the metal dusting reaction. (C) 2009 Elsevier B.V. All rights reserved.