Using Si2O2 rhombuses and Si3O3 rings as building blocks, we have investigated the structures of medium-sized silicon oxide clusters. Total energies of a series of Si8On and Si12On clusters with different growth motifs have been studied using first-principles calculations. Our studies discover a new motif for medium-sized silicon oxide clusters in a form of wheel-like structure composed of Si3O3 rings. (C) 2003 Elsevier B.V. All rights reserved.