IR changes caused by photolysis of CO from the mixed valence form of bovine cytochrome c oxidase have been investigated over the pH/pD range 6-9.8. Band assignments were based on effects of H2O/D2O exchange and by comparisons with published IR data and crystallographic data. Changes arise both from CO photolysis and from subsequent reversed electron transfer from heme a 3 to heme a. This reversed electron transfer is known to have pH-independent and, above pH 8, pH-dependent components. The pH-independent component is associated with a trough around the 1742 cm(-1) band attributable to one or more protonated carboxylic acids. Its peak position, but not extent, is pH-dependent, indicative of a titratable group with a p K of 8.2 whose acid form causes increased hydrogen bonding to the IR-detectable carboxylic group. A different protonatable group with p K above 9 controls the extent of the pH-dependent component. This phase is associated with perturbation of an arginine guanidinium that is most clearly observed as a trough at 1592 cm(-1) after H/D exchange. It is suggested that this group, probably Arg-438 that is in close contact with propionate groups of both hemes and already proposed to be of functional significance, lowers the energy of the transient charge-uncompensated electron-transfer intermediate by changing the charge distribution in response to heme-heme electron transfer. No other IR signature of a titratable group that controls the extent of the pH-dependent phase is present, and it most likely arises from a nonphysiological deprotonation of the proximal water ligand of ferric heme a 3 at high pH that has been reported to exhibit a similar pK.