A mechanistic study of the stoichiometric and catalytic H/D exchange reactions involving cationic iridium complexes is presented. Strong evidence suggests that both stoichiometric and catalytic reactions proceed via a monohydrido-iridium species. Stoichiometric deuterium incorporation reactions introduce multiple deuterium atoms into the organic products when aryliridium compounds Cp*PMe3Ir(C6H4X)(OTf) (X = H, o-CH3, m-CH3, p-CH3) react with D-2. Multiple deuteration occurs at the unhindered positions (para and meta) of toluene, when X = CH3. The multiple-deuteration pathway is suppressed in the presence of an excess of the coordinating ligand, CH3CN. The compound Cp*PMe3\rH(OTf) (1-OTf) is observed in low-temperature, stoichiometric experiments to support a monohydrido-iridium intermediate that is responsible for catalyzing multiple deuteration in the stoichiometric system. When paired with acetone-d(6), [Cp*PMe3IrH3][OTf] (4) catalytically deuterates a wide range of substrates with a variety of functional groups. Catalyst 4 decomposes to [Cp*PMe3Ir(eta(3)-CH2C(OH)CH2)][OTf] (19) in acetone and to [Cp*PMe3IrH(CO)]-[OTf] (1-CO) in CH3OH. The catalytic H/D exchange reaction is not catalyzed by simple H+ transfer, but instead proceeds by a reversible C-H bond activation mechanism.