In contrast to its lethargy at physiological pH, horse heart cytochrome c can be oxidized at room temperature by the axial inner sphere oxidant bromomalononitrile (BMN) at higher acidities. The following stoichiometry obtains: 2Fe11 c + BrCH(CN2) + H+ leads to 2FeIII c + CH2(CN)2 + Br-, and the rate law is given by: rate = k2(FeIIc)(BMN). At an ionic strength of 1.0 (KCl), second-order rate constants vary from 300 l. per mol per sec (pH 2-3) to 0(pH 9). Below pH 6 there is a noticeable increase in rate with ionic strength while there is no specific salt effect for the process. At pH 7.4 there is no influence of added salt (0.01-1.0 M) upon the slow rate of reaction. The vast changes in rate occur over a pH region (3-6) in which only very minor changes in the visible spectrum of the cytochrome are manifest. The results are interpreted in terms of a conformational isomerism of cytochrome c in which the effective redox geometry alters from a predominantly "short C" form (in which an axial position is available for substitution) at lower pH's to a predominantly "C" form (axial positions encumbered) in the physiological region. At 5 degrees, pH 7.4, both hemes of beef heart cytochrome oxidase are oxidized by the addition of BMN (k2 = 29 plus or minus 3 l. per mol per sec). However, the reaction is inhibited by potassium cyanide and the protein containing iron(II) cyt alpha along with the cyano adduct of iron(II) or iron(III) cyt alpha3 is inert. The results demonstrate cytochrome alpha3 as the site of reaction and that alpha reduces alpha3 in the process. Cytochrome oxidase does catalyze the oxidation of cytochrome c with BMN as substrate. Taken together the results provide additional support for a recent theory and they demonstrate BMN to be an efficient probe for the effective redox geometry of a hemoprotein in solution.