The octahedral alkoxo complexes mer-cis-HIr(OR)Cl(PR(3)’)(3) (R = Me, Et, i-Pr; R’ = Me, Et; H trans to Cl) decompose at room temperature in an alcohol/benzene solution, forming the dihydrido products mer-cis-H2IrCl(PR(3)’)(3) and the corresponding aldehyde or ketone, The reaction rate is of first order in the iridium complex and of 1.33 order in the alcohol, which serves as a catalyst. The rate depends on the nature of the phosphine (PEt(3) > PMe(3)), on the alkyl substituent of the alkoxide (Me > Et much greater than i-Pr), and on the medium (benzene > N-methylpyrrolidone) but is not effected by excess phosphine. The activation parameters obtained for the decomposition of mer-cis-HIr(OCH3)Cl(PMe(3))(3) are Delta H-obs(double dagger) = 24.1 +/- 1.8 kcal mol(-1), Delta S-obs(double dagger) = 0.6 +/- 5.9 eu, and Delta G(obs)(double dagger) (298 K) = 23.9 +/- 3.6 kcal mol(-1). The kinetic isotope effect (combined primary and secondary effects) for the decomposition of mer-cis-DIr(OCD3)Cl(PMe(3))(3) at 22 degrees C is k(H)/k(D) = 2.45 +/- 0.10, and the secondary kinetic isotope effect for the decomposition of DIr(OCH3)Cl(PMe(3))(3) at 22 degrees C is 1.10 +/- 0.06. Both DIr(OCH3)Cl(PMe(3))(3) and HIr(OCD3)Cl(PMe(3))(3) produce only the two mer-cis isomers of HDIrCl(PMe(3))(3), but in different ratios. The following steps are involved in the beta-hydride elimination process : (a) pre-equilibrium generation of a free coordination site by chloride dissociation, which is induced by hydrogen bonding of a methanol molecule to the chloride; (b) irreversible rate-determining beta-C-H cleavage through the sterically favored transition state; (c) facile, irreversible dissociation of the aldehyde; (d) ligand rearrangement; and (e) irreversible reassociation of the chloride. Selective deuterium labeling enables the elucidation of a competing minor mechanism through the electronically favored transition state, operative for the trimethylphosphine complex only.