Abstract 1. 1. In the absence of cytochrome c, the haem groups of cytochrome aa 3 titrate as indistinguishable identities, each having E′ o = 280 mV and n = 1. 2. 2. In the presence of cytochrome c, 45% of the absorbance change on reduction at 605 nm is due to a high-potential component with E′ o = 335–360 mV, n = 1.2 and 55% to a low-potential component with E′ o = 200–250 mV, n = 1.0. At 445 nm both components contribute equally to the difference spectrum (reduced minus oxidized).