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Adsorption and reaction of acetylene on a hexagonally reconstructed (5 × 20)-Pt(100) surface and two ordered Sn/Pt(100) alloy surfaces were investigated using temperature programmed desorption spectrometry (TPD), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). Vapor deposition of Sn onto a Pt(100) single-crystal substrate was used to form two Pt-Sn alloys, the c(2 × 2) and (3 2× 2)R45°Sn/Pt(100) structures with θ Sn ) 0.5 and 0.67 ML, respectively, depending on the initial Sn concentration and annealing temperature. Acetylene nearly completely decomposed during TPD on Pt(100) in the absence of Sn, forming hydrogen, which then desorbs as H 2 , and surface carbon. This decomposition, associated with irreversible dissociative adsorption, was strongly suppressed on the two Pt-Sn alloy surfaces, and a large acetylene desorption peak in TPD was observed. Additionally, 15% of the adsorbed acetylene monolayer was converted to gaseous benzene during TPD on the (3 2× 2)-R45°Sn/Pt(100) alloy. No such benzene desorption occurred from the c(2 × 2) alloy. Alloyed Sn in the c(2 × 2) alloy decreased the initial sticking coefficient of acetylene on Pt(100) at 100 K by ∼40%, but additional Sn in the other alloy had no additional effect. The saturation coverage of C 2 D 2 in the chemisorbed monolayer at 100 K decreased from that on Pt(100) by 35% on the c(2 × 2) alloy and 50% on the (3 2× 2)-R45°Sn/Pt(100) alloy. However, the c(2 × 2)-Sn adlayer eliminates acetylene chemisorption, illustrating that the effectiveness of Sn to "block" sites depends crucially on its location as an adatom or alloyed atom on Pt surfaces. The acetylene chemisorption bond energy, estimated by the acetylene desorption activation energy measured in TPD, also decreased (45-65%) as the alloyed Sn concentration increased. Multiple TPD peaks for C 2 D 2 desorption from both the c(2 × 2) and the (3 2× 2)R45°Sn/Pt(100) alloy surfaces indicate either several energetically distinguishable adsorption sites for acetylene or the rate-limiting influence of more complex surface reactions on these surfaces. † Part of the special issue "John T. Yates, Jr. Festschrift".