High levelab initiocalculations carried out for the2A′ and2A″ states of ArNO(X2Π) predict a crossing near the T-shape configuration, with the2A′ minimum being slightly deeper. Spin-orbit coupling is included through a model treatment and results in two potential energy surfaces with similar topologies, nearly parallel to each other and close to the averaged non-relativistic surface. These results are used to construct a DIM-like model for ArnNO clusters. The lowest energy cluster structures are found to resemble those for Arn+1with NO lying in the surface. The set of major magic numbers (structures of pronounced stability) is also the same as for the Arn+1clusters, and is emphasized further by the detachment of NO, which requires a larger energy than for detachment of a single Ar atom. The relations of the difference between the two dissociation energies and of the ArnNO(1/2 → 3/2) excitation energy to the magic numbers are discussed.