Coproferrihaem (cfh) reacts very rapidly with peroxybenzoic acids at pH = 6.0 in aqueous solution, in processes of 1 : 1 stoicheiometry, to form a spectroscopically distinct oxidised derivative coprohaem Compound I (ch-I), which is stable on the stopped-flow time-scale. The rate constants for ch-I formation are comparable to those for the formation of horse radish peroxidase Compound I with peroxybenzoic acids. The Soret band absorption spectrum of ch-I is independent of initial [cfh] and [peroxyacid] with R, the ratio [peroxyacid] : [cfh], in the range 1 < R < 4 and independent of the nature of the peroxyacid. Although cfh exists as >99% monomer under the conditions employed, the absorption spectra at R < I suggest limited dimer formation between ch-I and unoxidised cfh. Coprohaem Compound I exhibits powerful peroxidatic oxidant properties. In particular ch-I is reduced by methanol, ethanol, and propargyl alcohol (prop-2-yn-1-ol) with rate constants that parallel in sequence those with catalase Compound I, although the enzymatic intermediate reactions are 10²–10⁴ times faster than those with ch-I.