The adsorption and thermal decomposition of C2N2 on Si(100)-2 x 1 have been investigated with TPD, XPS, UPS, and HREELS. For the C2N2 species, two desorption peaks at 160 and similar to 230 K were noted in the TPD spectrum, deriving from the C2N2 adsorbed in overlayers and the submonolayer, respectively.;The latter showed C-1s and N-1s XPS peaks at similar to 1.8 eV lower in binding energy than those of the multilayer C2N2 The large energy shift in the XPS peaks is likely caused by the strong interaction between the C2N2 adsorbed side-on and the surface at lower coverages. Annealing the C2N2-doses Si(100) at similar to 600 K caused the breaking of the NC-CN bond, and the CN adspecies could be identified on the surface. At 700-800 K, CN bond scission occurred, and thus a mixture of silicon carbide and silicon nitride was formed. Similar results were found for C2N2 on Si(111)-7 x 7; however, the relative signal intensities, e.g., C-1s and N-1s XPS and HREELS, are weaker than those of the C2N2 On Si(100). This difference may be attributable to the fact that there are fewer dangling bonds on Si(ll 1)-7 x 7 than on Si(100)-2 x 1.