Photoreaction, time-resolved infrared (TRIR), and DFT studies were utilized to probe transformations between iridium complexes with possible relevance to the mechanisms of the iridium/iodide-catalyzed methanol carbonylation to acetic acid. Solution-phase continuous and laser flash photolysis of the tetraphenylarsonium salt of the fac-[CH3Ir(CO)(2)I-3](-) anion (1a) under excess carbon monoxide resulted in migratory insertion to give the acyl complex ion mer,trans-[Ir(C(O)CH3)(CO)(2)l(3)]- (2a). The latter was isolated as its AsPh4+ salt, and its X-ray crystal structure was determined. TRIR spectra indicate that several transients are generated upon flash photolysis of la. The principal photoreaction is CO dissociation, and this is proposed to generate the isomeric complexes fac-[CH(3)1lr(CO)(Sol)l(3)](-) (I-CO(fac), Sol = solvent) and mer,trans-[CH(3)lr(CO)(Sol)l(3)]- (I-CO(mer)). I-CO(fac) reacts with CO to regenerate 1a with a second-order rate constant (k(CO)) similar to 2.5 x 10(7) M-1 s(-1) in ambient dichloroethane, while I-CO(mer) is the apparent precursor to 2a. Kinetics studies indicate the photoinduced formation of a third intermediate (I-M), hypothesized to be the anionic acyl complex fac-[lr(C(O)CH3)(CO)(Sol)l(3)](-). In the absence of added CO, these intermediates undergo dimerization to form a mixture of isomers with the apparent formula [lr(C(O)CH3)(CO)l(3)](2)(2-). One of these dimers was isolated as the AsPh4+ salt, and the crystal structure was determined. Addition of excess pyridine to a solution of the dimers gave the neutral complex mer,trans-[lr(C(O)CH3)(CO)(py)(2)l(2)], which was characterized by FTlR, NMR, and X-ray crystallography. These transformations, especially the unprecedented photoinduced CO insertion reaction, are discussed and interpreted in terms of the factors favoring migratory insertion dynamics.