Gas-phase MxSy+ (M = Mo, W) cluster ions were generated by reactive sputtering using a magnetron cluster source. Adsorption of CO and NH3 were used to confirm the stoichiometry of the more prominent MxSy+ (M = Mo, W) clusters. For the most abundant M4S6+ cluster, the adsorption experiments are consistent with a calculated structure in which the metal atoms are arranged in an internal tetrahedron surrounded by bridging S-atoms. Binding energies for the consecutive addition of CO and NH3 were also calculated and found to be consistent with the observed cluster adduct stability. (c) 2005 Elsevier B.V. All rights reserved.