The ferric forms of Met80X mutants of yeast iso-1-cytochrome c (X = Ala, Ser, Asp, and Glu) display EPR and optical spectra that are strongly pH dependent. At low pH values (pH approximate to 5) the sixth coordination sites are filled by H2O that, on elevating the pH, is replaced by OH- in the cases of Met80Ala and -Ser (pKapproximate to 5.6 and 5.9, respectively) and by a lysine amino group in the cases of Met80Asp and -Glu (pK approximate to 9.3 and 11.6, respectively). The ligand sets and the pK values of the transitions are rationalized in terms of the structure of the heme pocket, and a possible mechanism of the "trigger"in the alkaline transition of the native protein is suggested.