The gas phase reaction of Th+ with H2O to produce HThO+ H and ThO+ + H-2 was investigated using density functional theory and coupled cluster methods. RRKM calculations of the branching ratio favor the H atomic elimination channel in disagreement with experiment Ab initio classical trajectory calculations were carried out to obtain a better model of the molecular dynamics. The molecular dynamics simulations yield a branching ratio of ca. 80% for the H-2 elimination channel to 20% for the H atomic elimination channel in qualitative agreement with the observed ratio of 65% to 35%.