The adsorption of NO on the Si(111)-7 x 7 surface has been investigated by means of hybrid density functional (B3LYP) cluster model calculations. It is shown that both monomeric and dimeric adspecies can be formed upon NO adsorption. The monomeric adspecies adopt bent configurations with the N-end of the admolecule forming a covalent N-Si bond with a surface atom (adatom or restatom). Three dimeric adspecies have been found either adopting cis-N2O2 configuration or trans-N2O2 configurations with rather short N=N bond length. These dimeric adspecies should be subject to further decomposition to eliminate N-2 or N2O. (C) 2003 Elsevier Science B.V. All rights reserved.