Spectroscopic studies of the adsorption of dimethyl sulfoxide, (CH3)2S = O, on Pt(111) have shown that the molecule is bound to the surface via the sulfur atom in an inverted pyramid configuration. A comparison of XPS and EELS data for the adsorbed multilayer and monolayer with XPS and infrared data on the complex PtCl2(DMSO)2 is consistent with sulfur bonding. In addition, we detect a considerable increase of the v(S=O) frequency in the DMSO monolayer with decreasing coverage, indicating a coverage dependent heat of adsorption. UPS data show that on adsorption to form a monolayer, the highest occupied molecular orbital of DMSO, presumably the sulfur "lone pair" orbital, shifts to a higher binding energy. These results show a remarkable similarity between DMSO bonding to a metal surface and bonding to a single Pt2+ species.