Transition metal homogeneous catalysts active for liquid-phase dehydrogenation of methanol to yield formaldehyde and dihydrogen under refluxing conditions have been found, in addition to those active only under photoirradiation. From the mixture of [Ru 2(OAc) 4Cl] and PR 3 dissolved in methanol, the reactive complex of [Ru(OAc)Cl(PPh 3) 3] or [Ru(OAc)Cl(PEtPh 2) 3] was isolated and characterized. The addition of acetic acid enhanced the initial rate, suggesting the proton-hydride reaction scheme. The catalyst complex, [Ru(OAc)Cl(PEtPh 2) 3], was deactivated gradually, with the bidendate OAc ligand being displaced with H and CO. The mechanism of the C—H cleavage of a methoxy ligand to give HCHO and an intermediate hydride complex, occurring on these cis-disposed sites, and the physico-chemical properties of metal complexes appropriate to methanol dehydrogenation are discussed.