By means of scanning tunneling microscopy (STM), it has been possible to obtain the first atomic-scale images of the Co-Mo-S structure present in hydrodesulfurization (HDS) catalysts. Information on the catalytically important edge structures has been obtained by synthesizing single-layer Co-Mo-S nanoclusters using the Au(III) herringbone structure as a template, It is observed that the presence of the Co promoter atoms causes the shape of the MoS2 nanoclusters to change from triangular to hexagonally truncated. This change in morphology appears to be driven by a preference for Co to be located at the S-edge of MoS2. The results also directly show that the presence of the Co atoms perturbs the local electronic environment of neighboring S atoms and this provides further insight into the effect of the promoter atoms. (C) 2000 Academic Press.