1. The absorption spectra of deutero- and proto-ferrihaem in aqueous solution at 25°C show marked changes with concentration and pH in the Soret band region. Quantitative studies of these phenomena imply that they are associated with ferrihaem dimerization and with protolytic equilibria involving monomeric (M) and dimeric (D) ferrihaem species according to the scheme: [Formula: see text] 2. For deuteroferrihaem we obtain K=1.9×10−2, pKa(M)=7.1, pKa(D)=7.4. Protoferrihaem has a much higher dimerization constant, K=4.5 and pKa(D)=7.5 (pKa(M) is not accessible). 3. Possible structural relationships between monomeric and dimeric ferrihaem species in solution are discussed in relation to recent work on the oxo-bridged nature of crystalline ferrihaem dimers.

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