Complex formation between ferricytochrome c peroxidase (CCP) and ferricytochrome c from yeast [cyt(Y)] and horse heart [cyt(H)] was studied by resonance Raman spectroscopy. On the basis of a detailed spectral analysis of the free proteins, it was possible to attribute changes in the spectra of the complexes to the individual proteins. At pH 7.0 both cyt(Y) and cyt(H) binding induces an increase in the six-coordinate low-spin configuration of CCP from 9% to 19% at the expense of the five-coordinate high-spin state, which drops from 84% to 74%. In the free and complexed state, CCP exhibits a constant fraction of the six-coordinate high-spin form (approximately 7%). In addition to affecting the coordination state, there is also a cyt-specific structural response of CCP to complexation. In the cyt(Y)-CCP complex, the peripheral vinyl and propionate substituents of CCP are more rigidly fixed in the protein matrix, whereas binding of cyt(H) only slightly perturbs the conformations of these side chains. The biological significance of the conformational changes in CCP are discussed. In contrast to CCP, there are no detectable structural changes in either cyt(Y) or cyt(H) upon complex formation.