Adsorption and decomposition of cyclohexanone (C6H10O) on Pt(111) and on two ordered Pt-Sn surface alloys, (2 x 2)-Sn/Pt(111) and (root 3 x root 3)R30 degrees-Sn/Pt(111), formed by vapor deposition of Sn on the Pt(111) single crystal surface were studied with TPD, HREELS, AES, LEED, and DFT calculations with vibrational analyses. Saturation coverage of C6H10O was found to be 0.25 ML, independent of the Sit surface concentration. The Pt(111) surface was reactive toward cyclohexanone, with the adsorption in the monolayer being about 70% irreversible. C6H10O decomposed to yield CO, H2O, H-2, and CH4. Some C-O bond breaking occurred, yielding H2O and leaving some carbon on the surface after TPD. H REELS data showed that cyclohexanone decomposition in the monolayer began by 200 K. Intermediates from cyclohexanone decomposition were also relatively unstable on Pt(111), since coadsorbed CO and H were formed below 250 K. Surface Sn allowed for some cyclohexanone to adsorb reversibly. C6H10O dissociated on the (2 x 2) surface to form CO and H2O at low coverages, and methane and H-2 in smaller amounts than on Pt(111). Adsorption of cyclohexanone on (root 3 x root 3)R30 degrees-Sn/Pt(111) at 90 K was mostly reversible. DFT calculations suggest that C6H10O adsorbs on Pt(111) in two configurations: by bonding weakly through oxygen to an atop Pt site and more strongly through simultaneously oxygen and carbon of the carbonyl to a bridged Pt-Pt site. In contrast, on alloy surfaces, C6H10O bonds preferentially to Sn. The presence of Sn, furthermore, is predicted to make the formation of the strongly bound C6H10O species bonding through O and C, which is a likely decomposition precursor, thermodynamically unfavorable. Alloying with Sn, thus, is shown to moderate adsorptive and reactive activity of Pt(111).