The respiratory heme-copper oxidases catalyze the reduction of dioxygen to water and link this chemistry to proton translocation. The main subgroups of the enzyme family are the cytochrome c oxidases and the quinol oxidases. For the cytochrome c oxidases, several key intermediates have been described in the oxygen reaction. Two of these (suggested to be "peroxy" and "ferryl" species) are also produced in the reaction of the oxidized enzyme with hydrogen peroxide. However, only a single product (a "ferryl" species) has been reported for the reaction of hydrogen peroxide with the quinol oxidase cytochrome bo3 from Escherichia coli. The same "ferryl" species has also been reported to be produced when two-electron reduced cytochrome bo3 reacts with oxygen, whereas this reaction leads to the "peroxy" intermediate in the cytochrome c oxidases. Consequently, the oxygen reaction has been considered to be different in the two enzyme subgroups. Here we show that both the peroxide reaction and the reaction of the two-electron reduced enzyme with oxygen actually result in primary formation of a hitherto unreported "peroxy" species in cytochrome bo3. This intermediate subsequently relaxes into the "ferryl" species which has been described previously. We conclude that the oxygen reaction is similar in the cytochrome c and quinol oxidases.