Although it is hardly ever explicitly discussed, slab-like supported MoS2 catalysts are not stable upon exposure to air. Transmission electron microscopy studies show that for supported MoS2, prepared using atmospheric pressure gas-phase sulphidation, particle size decreases upon exposure to ambient air for 24 h, whereas after 1 month of air exposure the number of slabs remaining has decreased significantly. For 40 bar liquid-phase sulphided supported MoS2, a significant decrease in particle size is observed already after 5 min exposure to ambient air. (C) 2007 Elsevier B.V. All rights reserved.