Rearrangements on the ground-state singlet Ga2H2 surface leading from the most stable planar dibridged Ga(mu-H)(2)Ga isomer 1 to the branched GaGaH2 2 and trans HGaGaH 3 have been studied by using correlated ab initio and DFT methods. The energetically favourable rearrangements can be achieved via a two-step mechanism involving the monobridged HGa(mu-H)Ga isomer 4 as intermediate. The transition states involved in the two-step pathways are predicted to lie ca. 10-15 kcal/mol above 1. A new type 'butterfly' Ga(mu-H)(2)Ga species 5, found in this work, would rearrange with no barrier to 1. (C) 2003 Elsevier B.V. All rights reserved.