High-resolution scanning tunneling microscopy (STM) is used in combination with density-functional theory (DFT) to provide new insight into the morphology and atomic-scale structure of MoS2 nanoclusters in hydrodesulfurization (HDS) catalysts. Atom-resolved STM images of gold-supported single-layer MoS2 nanoclusters reveal the first direct evidence that both the detailed atomic-scale structure of the catalytically important edges and the overall morphology of the nanoparticles are sensitive to sulfiding and reaction conditions. Specifically, it is shown that synthesis in H2S:H-2 = 500 results in MoS2 nanoclusters with a triangular morphology, whereas sulfiding in H2S:H-2 = 0.07 leads to hexagonally truncated nanoclusters. For both morphologies we identify the exact geometric edge structure of the MoS2 nanoclusters by comparing the atom-resolved STM images with STM simulations. Whereas the MoS2 triangles are terminated by dimer-saturated Mo edges, the hexagonal MoS2 structures exhibit completely different edge structures with a lower sulfur coverage on the Mo edges and S edges with adsorbed S-H groups. A thermodynamic model based on DFT is employed to construct phase diagrams which can predict the stability of different MoS2 edge structures under different conditions. The present results thus provide new insight into the atomic structure of the HDS catalysts and how it may change with reaction conditions. (C) 2003 Elsevier Inc. All rights reserved.