The density functional theory (DFT) method has been used to investigate NO probe molecule adsorption on the stoichiometric (Mo(16)S(32)) and nonstoichiometric (Mo(16)S(34) and Mo(16)S(29)) clusters. The calculated adsorption energies indicate that the stoichiometric cluster has stronger NO affinity than the nonstoichiometric surfaces. It is also found that mononitrosyl adsorption is favored at low NO coverage, while dinitrosyl (germinal) and (NO)(2) dimer adsorption at high NO coverage are possible. Strong repulsive interaction has been found for the adsorbed dinitrosyl and (NO)(2) dimer species. In addition, the computed NO stretching frequencies for the mononitrosyl and dinitrosyl species agree well with the experimental data, while those of the dimer species are much lower than the suggested experimental data.