Cyanide reacts with oxidized cytochrome oxidase in intact mitochondria to form a low-spin cyanide compound. The reaction is first order in time but obeys Michaelis-Menton kinetics with respect to cyanide concentration. The cyanide concentration required for one-half maximal rate is approximately 4 μ m in uncoupled mitochondria at pH 6.5. A pH dependent equilibrium exists between two forms of oxidized cytochrome a 3 (p K 6.9) and the pH dependence of the cyanide reaction is interpreted as arising from its specific reaction with the acid form (but not with the base form). The initial cyanide complex is spectroscopically undetected and the measured cyanide reaction represents the rate of conversion from the intermediate form ( K d = 4 μm) to the spectroscopically distinct form (overall K d < 0.2 μm). The second order rate constant of the cyanide reaction with oxidized cytochrome oxidase is approximately 1 × 10 4 m −1 sec. −1 Cyanide also binds to reduced cytochrome oxidase in intact mitochondria with a dissociation constant of 300 μ m.