The geometries and the vibrational frequencies for several SiSO+ and GeSO+ species at doublet state have been predicted at density functional theory level with a 6-311+G* basis set. The detailed bonding character is discussed, and the state-state energy separations of various stable states relative to the ground state are calculated. The ground states are linear Si-OS+ and cyclic GeSO+ for two systems, respectively. Result analysis indicates that the (2)A" cyclic state should be classified as the thiosuperoxide, the bent structure and the linear M-OS+ structure have some thiosuperoxide characters, but the linear S-M-O+ may be classified as thio-oxide. (C) 2003 Published by Elsevier B.V.