Amorphous MoS3 (air-dried precipitate), crystalline MoS2 (made in the laboratory under nitrogenous atmosphere), and commercial molybdenum disulfide catalysts have been investigated by X-ray photoelectron spectroscopy (XPS). The chemical species present on the surface and the S/Mo atomic ratios of laboratory catalysts were compared to those present on a commercial molybdenum disulfide catalyst before and after treatment with CO and H-2. Pretreatment Of MoS2 with CO followed by H-2 showed that CO was adsorbed on the surface with no change in the chemical oxidation state of Mo and S. However, the results indicated that CO was not adsorbed when the sequence of gas exposures was reversed. (C) 2003 Elsevier Inc. All rights reserved.