Mechanistic studies were conducted on beta -hydrogen elimination from complexes of the general formula [Ir(CO)(PPh3)(2)(OR)], which are square planar alkoxo complexes with labile ligands. The dependence of rate, isotope effect, and alkoxide racemization on phosphine concentration revealed unusually detailed information on the reaction pathway. The alkoxo complexes were remarkably stable, including those with a variety of electronically and sterically distinct groups at the beta -carbon. These complexes were much more stable than the corresponding alkyl complexes. Thermolysis of these complexes in the presence of PPh3 yielded the iridium hydride [Ir(CO)(PPh3)(3)H] and the corresponding aldehyde or ketone with rate constants that were affected little by the groups at the beta -carbon. The reactions were first order in iridium complexes. At low [PPh3], the reaction rate was nearly zero order in PPh3, but reactions at high [PPh3] revealed an inverse dependence of reaction rate on PPh3. The rate constants were similar in toluene, THF, and chlorobenzene. The y-intercept of a 1/k(obs) vs [PPh3] plot displayed a primary isotope effect, indicating that the y-intercept did not simply correspond to phosphine dissociation. These data and a dependence of alkoxide racemization on [PPh3] showed that the elementary beta -hydrogen elimination step was reversible. A mechanism involving reversible beta -hydrogen elimination followed by associative displacement of the coordinated ketone or aldehyde by PPh3 was consistent with all of our data. This mechanism stands in contrast with the pathways proposed recently for alkoxide beta -hydrogen elimination involving direct elimination, protic catalysts, or binuclear mechanisms and shows that alkoxide. elimination can follow pathways similar to those for,beta -hydrogen elimination from alkyl complexes.