We dedicate this article to Helmut Siegel for his enormous contributions to Bioinorganic Chemistry (BIC) and, together with his wife Astrid, had an instrumental role in the construction of a friendly BIC community. AbstractRedox titrations followed by visible spectroscopy were performed at various pH values with the nine-haem cytochrome c from Desulfovibrio desulfuricans ATCC 27774 (Dd 27774). Macroscopic midpoint reduction potentials of the haems and pK a ox and pK a red values of acid Á/base centres that interact with the redox centres were estimated by using a thermodynamic model, in which the haems are grouped in three sets, and two acid Á/base centres are considered. The results show that this cytochrome displays redox-Bohr effects in the physiological pH range and that electrostatic effects dominate the macroscopic interactions between the acid Á/base and redox centres. #