The heme-associated, pH-induced transition in iodinated ferricytochrome c has been studied by electron paramagnetic resonance (E.P.R.) and absorption spectroscopy. The conversion from type III to type IV (as defined by Theorell, H., and Åkesson, Å.: J. Am. Chem. Soc. 63, 1812 (1941)) ferricytochrome c forms was dramatically altered by iodination of tyrosyl residues. Both absorption and E.P.R. spectra suggested that a transition between similar heme-iron coordination structures occurred with a pK of about 6 as compared with about 9 for native ferricytochrome c. At pH 4.4 the E.P.R. spectrum (liquid nitrogen temperature) of a frozen solution of iodinated ferricytochrome c was similar to the native type III at pH 7, except for an increased g = 6.0 signal from high-spin heme iron. At neutral pH the E.P.R. spectrum of the iodinated derivative was similar to type IV ferricytochrome c. The results give further support to the hypothesis that the pK of tyrosine 67 plays an important role in determining the pK of the III to IV transition.