The adsorption of NO and CO on a nonhomogeneous Pt(100) surface made up of both the (hex) and (1 x 1) structural phases was studied at 300 K by means of high-resolution electron energy-loss spectroscopy (HREELS). The nonhomogeneous surface was prepared by titrating NOads/(1 x 1) islands formed on the original (hex) surface with deuterium and subsequent heating to desorb residual D-ads. The as-prepared surface was assumed to consist of patches of the (1 x 1) structure surrounded by the (hex) surface; The ratio between the (1 x 1) patches and the (hex) area was controlled by the initial coverage of NOads before titration. The exposure of the nonhomogeneous surface to NO or CO at 300 K led to saturation of the (1 x 1) patches first. It was supposed that the NO and CO molecules adsorbing on the (hex) areas quickly diffuse along the surface, meet the (1 x 1) patches, and are "trapped" by them. NOads (COads) spreads on the surface of the (1 x 1) patches. As soon as the (1 x 1) patches are saturated, the adsorption-induced (hex) --> (1 x 1) back-reconstruction takes place. The details of the NO and CO adsorption on the nonhomogeneous surface were compared with the process on the reconstructed and unreconstructed Pt(100) surfaces.