Sum frequency generation (SFG) vibrational spectroscopy is used to study the adsorption of CO at similar to 1 atm pressure on Pt nanoparticle arrays and Pt thin films. The SFG signal of CO adsorbed on platinum particles of 45 nm diameter is similar to 10 000 times larger than from CO on smooth Pt films. The large enhancement is explained by plasmon resonance and Maxwell-Garnett theory. The Pt arrays are prepared using electron beam lithography to produce particles with uniform spacing and sizes on an oxidized Si(100) wafer. Further, as the Pt coverage increases the SFG signal shows a polarization dependence that is explained considering the dielectric properties of a metal film on a dielectric surface. In addition, SFG permits investigation of the CO adsorbed on the particles at similar to 1 atm, which is not possible with most surface analytical techniques, that will allow for the study of the reaction of small molecules on surfaces relevant in heterogeneous catalysis.