Purified bovine heart cytochrome c oxidase (CcO) has been extracted from aqueous solution into hexane in the presence of phospholipids and calcium ions. In extracts, CcO is in the so-called "slow" form and probably situated in reverse micelles. At low water:phospholipid molar ratios, electron transfer from reduced heme a and Cu(A) to the catalytic center is inhibited and both heme a3 and Cu(B) remain in the oxidized state. The rate of binding of cyanide to heme a3 in this oxidized catalytic center is, however, dependent on the redox state of heme a and Cu(A). When heme a and Cu(A) are reduced, the rate is increased 20-fold compared to the rate when these two centers are oxidized. The enhanced rate of binding of cyanide to heme a3 is explained by the destabilization of an intrinsic ligand, located at the catalytic site, that is triggered by the reduction of heme a and Cu(A).