We have detected, using electrospray mass spectrometry, minor changes in the H/D exchange rates in various solvents for cytochromes c obtained from five different species. We compared the exchange rates exhibited by these proteins by mixing horse cytochrome c with each of the other four species and monitoring their exchanges simultaneously by mass spectrometry. The use of horse cytochrome c as a reference allowed us to make very accurate comparisons of the small differences in hydrogen exchange rates among the various species. The exchange experiments were performed in water and methanol at several concentrations in an effort to determine whether the cytochromes c of these five species have different conformations in specific solvents, which would cause their exchange rates to differ. Therefore, monitoring the level of exchange as a function of time in both water and water-methanol mixtures is a method for detecting subtle structural changes of proteins in their native or unfolded intermediate states.