The reaction mechanism of the ground state (1)Sigma(+) of LaS+ with oxygen-transfer reagent: LaS+ + H2O --> Lao(+) + H2S in the gas phase has been proposed and investigated by ab initio methods. The reaction is proceeding via two 1,3-hydrogen-shift reactions from oxygen atom to sulfur atom (via TS1 and TS2). The activation energies of two steps are 20.8 and 29.5 kcal/mol. respectively, at MP4 (SDTQ)/6-31G(++)//MP2/6-31G(++) level plus zero-point energy relative to their corresponding intermediate reactants. But TS1 locates higher in energy that TS2 by only 1.8 kcal/mol on the reaction path. The periodic trends of Sc, Y and La for this type of reaction were also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.