A combination of experiment and density functional theory was used to investigate the energetics of CO adsorption onto several small MxSy+ (M = Mo, W; x/y = 2/6, 3/7, 5/7, 6/8) clusters as a probe of their atomic and electronic structure. Experimentally, tandem mass spectrometry was used to measure the relative yields of MxSy+(CO)(n) cluster adducts formed by collisions between a beam of mass-selected MxSy+ cluster ions and CO molecules in a high-pressure collision cell (hexapole ion guide). The most probable MxSy+(CO)(n) adducts observed are those with n <= x, that is, only one CO molecule bound to each metal site. The notable exception is the M5S7+ cluster, for which the n = 6 adduct is found to have nearly the same intensity as the n = x = 5 adduct. Density fuctional calculations were used to search for the lowest energy structures of the bare MxSy+ clusters and to obtain their relative stability for sequential CO binding. The calculated trends in CO binding energies were then compared to the experimental adduct distributions for assigning the ground-state structures. In this way, it was possible to distinguish between two nearly isoenergetic ground-state isomers for the M2S6+ and M3S7+ clusters, as only one isomer gave a calculated CO stabilization energy trend that was consistent with the experimental data. Similar comparisons of predicted and observed CO adsorption trends also provide evidence for assigning the ground-state structures of the M5S7+ and M6S8+ clusters. The latter contain metallic cores with most of the sulfur atoms bonded along the edges or in the faces of the metal core structure. The n = 6 and 7 adducts of M5S7+ are predicted to be more stable than the n = x = 5 adduct, but only the n = 6 adduct is observed experimentally. The DFT calculations show that the n = 7 adduct undergoes internal bond breaking whereas the n = 6 framework is stable, albeit highly distorted. For the M6S8+ cluster, the calculations predict that the two lowest energy isomers can bind more than six CO molecules without fragmentation; however, the apparent binding energy drops significantly for adducts with n > 6. In general, the ability of these small MxSy+ clusters to bind more CO molecules than the number of metal atoms is a balance between the gain in CO adsorption energy versus the strain introduced into the cluster structure caused by CO crowding, the consequences of which can be fragmentation of the MxSy+(CO)(n) cluster adduct (n > x).