Reduced minus aerated difference spectra of membranes from Escherichia coli (grown under oxygen-limited conditions) show, in addition to the 650 nm trough attributed to the oxygenated form of cytochrome d, a smaller trough centred at about 680 nm of unknown origin. When the reference spectrum is that of a sample oxidized with ferricyanide and to which hydrogen peroxide was added, the trough proportions changed, the 680 nm species being more dominant. Similarly, when 8.8 mM hydrogen peroxide is added to a persulphate-oxidized sample, a peak at 680 nm is immediately formed. No such compound is observed when peroxide is added to persulphate-oxidized membranes from a cytochrome d-deficient mutant. It is concluded that the 680 nm species represents a peroxy form of haem d, which is stable at room temperature and is probably an intermediate in the reaction mechanism of this oxidase.