We report the first IR characterization of a single C-D bond within a protein, methyl-d1 Met80 of horse heart cytochrome c. A comparison was made to methyl-d1/d3 methionine as well as methyl-d3 Met80. We found that for methyl-d1 and the asymmetric stretches of methyl-d3, line widths/line shapes are dominated by inhomogeneous broadening, whereas the symmetric stretch of methyl-d3 has a significant homogeneous component. Vibrational energy relaxation calculations found that a significantly stronger Fermi resonance exists for the symmetric stretch than for the asymmetric stretches, thereby suggesting that a difference in intramolecular vibrational relaxation (IVR) causes the observed line width/line shape difference between the symmetric and asymmetric stretches.