Cytochrome c′, a heme protein isolated from photosynthetic and denitrifying bacteria, has previously been shown to have unusual chemical and physical properties. EPR and magnetic susceptibility measurements reported here indicate that in the pH range 1-11, the oxidized form of the protein can exist in four magnetically distinguishable states. Reversible transitions between these states can be induced by changing the pH of the protein solution. The two protein states which exist at physiological pH have magnetic properties unlike any other known heme protein. We show that these unique magnetic properties can best be explained by postulating iron electronic states which are quantum mechanical admixtures of an intermediate spin state and a high spin state. The suggestion, previously made, that the unusual magnetic properties of the protein are due to a thermal mixture of high and low spin states, is shown to be inconsistent with the magnetic data. The protein states at pH 1 and 11, though slightly dissimilar in symmetry at the iron site, are both typical ferric high spin states, quite similar in magnetic properties to the acid forms of metmyoglobin and methemoglobin. In the pH range 1-11 small anions are unable to bind to the iron site, indicating the presence of a strong hydrophobic region in the vicinity of the heme. Possible ligand-iron configurations corresponding to the four different protein states are discussed. © 1974.