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Cytochrome f, a c-type cytochrome involved in the photosynthetic electron transport chain, has a significantly higher redox potential than most other c-type cytochromes, ranging +370 to +380 mV. Like cytochrome c, cytochrome f also exhibits an alkaline transition in which the redox potential becomes pH dependent at high pH. In the case of cytochrome c, this has been attributed to replacement of the methionine sulfur serving as the sixth iron ligand by a deprotonated amino group. This cannot be the cause for the alkaline transition in cytochrome f as there is no methionine ligand to the iron, the sixth position being occupied by the N-terminal amino group. Three tyrosine phenolic groups (Y1, Y9, and Y160) are found in close proximity to the heme in the cytochrome f structure. To explore the possibility that the ionization of one or more of these tyrosines might be responsible for the alkaline transition, we have performed site directed mutagenesis, replacing each with a phenylalanine residue which lacks an ionizible group. Each of these mutants was found to have a redox potential of 375-380 mV at pH 7.0 which became pH dependent above pH 9.0 (apparent pKa 9.3). It thus seems unlikely that any of these tyrosine residues contributes to the alkaline transition of the redox potentials in cytochrome f. Redox properties of Y160L and R156L mutants will also be described.