Illumination of oxidized cytochrome oxidase with low intensity (<2 mW) light below 300 nm in the presence of oxygen causes pH-dependent spectral changes in the Soret and visible regions. The light-induced difference spectra show a peak at 438 nm and a trough at 414 nm in the Soret region and a peak at 606 nm and a shoulder at approximately 577 nm in the visible region. The effect was inhibited by cyanide, suggesting the involvement of cytochrome a3. The pH dependence indicates two titratable groups with pKa values of 6.52 +/- 0.26 and 6.85 +/- 0.15. The spectral changes are analogous to those occurring upon addition of hydrogen peroxide to the fully oxidized enzyme, which results in a mixture of species with absorbance maxima at 607 and 580 nm when referenced against the oxidized enzyme. Catalase addition affected the initial onset of the spectral change and increased the rate at which the reverse reaction occurred upon termination of illumination. The data are consistent with a mechanism involving light-induced autoreduction of the binuclear center and subsequent O2 binding, followed by the release of hydrogen peroxide and the formation of a mixture of the 607 nm and 580 nm forms.