Adsorption and reaction of NO on the (5 x 20)-Pt(100) surface and two Sn/Pt(100) surface allays have been studied using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). On the (5 x 20)-Pt(100) surface, in the absence of Sn, NO is primarily reversibly adsorbed, and most of the chemisorbed NO desorbs molecularly from the surface during TPD. Approximately 25% of the adsorbed NO monolayer decomposes at temperatures higher than 400 K, and this leads to N-2 and O-2 desorption from the surface. Alloying Sn into the surface layer of Pt(100) forms two ordered surface alloys having c(2 x 2) and (3 root 2 x root 2)R45 degrees Sn/Pt(100) surface structures with theta(sn) = 0.5 and 0.67 ML, respectively. Alloying reduced the saturation coverage of NO in the chemisorbed monolayer from that on Pt(100) at 100 K. and it also reduced the adsorption energy of molecularly bound NO by more than a factor of 2. Alloyed Sn, which removes all pure-Pt 2-fold bridge and 4-fold hollow sites, completely changed the NO reaction pathway: nitrogen in NO was partially reduced to form N2O on these alloys so that N2O along with NO and O-2, desorption was observed in TPD. NO is bonded at the same site with a similar geometry on both Pt(100) and the Sn/Pt(100) alloyed surfaces at low NO coverages, based on the HREELS spectra. At saturation (monolayer) coverages, however, quite different HREELS spectra were observed on both of the Sn/Pt(100) alloyed surfaces compared to that on Pt(100). Vibrations ware observed from adsorbed N2O. along with a shift of more than 60 cm(-1) for two nu(NO) peaks on both of the Sn/Pt(100) alloys compared to Pt(100). The two nu(NO) peaks can be assigned either as (i) two nu(s) modes of bent (tilted) and linearly bonded atop NO or as (ii) nu(s) and nu(as) stretching modes of a surface dinitrosyl species, that is, two NO molecules bound to one Pt atom. Dinitrosyl species have been proposed as intermediates for N2O formation in reactions of NO on Mo(110) (Queeney and Friend, J. Chem. Phys. 1997, 107, 6432), and we suggest that a similar reaction mechanism occurs on Sn/Pt(100) alloys.