An ab initio molecular orbital study is presented on the mechanism of the hydride exchange reaction in the trimetallic complex M(3)(CO)(9)(mu-H)(3)(mu(3)-CH) (M = Os, Ru). Based on the calculated structures and energies of many intermediates and transition states, it is proposed that the hydride exchange reaction takes place in multiple steps via two energetically competitive pathways. The "inner" pathway consists of the following steps : (1) migration of the bridging hydride H* to the nearby equatorial terminal position to form a seven-coordinated M; (2) clockwise turnstile rotation of three carbonyls and H* around this M; (3) insertion of H* into the M-C bond; (4) formation of the H*CH transition-state complex as the midpoint of the degenerate exchange process; and finally the reversal of steps 3, 2, and 1. The "outer" pathway involves the following steps : (1’) migration of the bridging H* to the farther-away equatorial terminal position to form a seven-coordinated M; (2’) anticlockwise turnstile rotation of three carbonyls and H*; and finally a merging with step 3 of the "inner" pathway. Some of the above steps may take place as a concerted single step. The calculated barrier heights are in good agreement with the experimental values and support the experiment in that the reaction of the Ru complex should be easier than that of the Os complex. A direct mechanism without involving seven-coordinated intermediates is unlikely.