The hydrogen-deuterium exchange rates of the reduced and oxidized forms of Rhodobacter capsulatus cytochrome c2 were studied by 1H-15N homonuclear multiple quantum correlation spectroscopy. Minimal differences were observed for the N- and C-terminal helices on changing redox state suggesting that although these helices are structurally important they do not affect the relative stability of the two redox states and hence may not be important in determining the redox potential differences observed amongst the class I c-type cytochromes. However, significant differences were observed for other regions of the protein. For example, all slow exchanging protons of the helix spanning Phe82 to Asp87 are similarly affected on reduction indicating that the unfolding equilibrium of this helix is altered between the two redox states. Other regions are not as simple to interpret; however, the difference in NH exchange rates between the redox states for a number of residues including His17, Leu37, Arg43, Ala45, Gly46, Ile57, Val58, Leu60, Gly61 and Leu100 suggest that interactions affecting the causes of these differences may be important factors in determining redox potential.