Atomic structures Of MonO2n, n = 1-6 clusters are studied using first principles calculations and are compared with those obtained for MonS2n clusters. The lowest energy isomers of MonO2n differ significantly from those of MonS2n for n > 3. Oxygen atoms favor to bind with two Mo atoms, while S atoms are bonded with three Mo atoms. However, the clustering of Mo, atoms is generally favored xin both cases due to the partially occupied d-orbitals. The lowest energy isomers of MonO2n have larger HOMO-LUMO gaps than the values for MonS2n clusters and for n = 1 and 4 the clusters are magnetic. (c) 2006 Elsevier B.V. All rights reserved.