We measured the temperature dependence of the zero-fluence extrapolated hole width associated with the S-1 <-- S-0 transition in protoporphyrin IX substituted myoglobin. The temperature was varied between 50 mK and 15 K. The experiments were done at two frequencies in the inhomogeneous band. We found that the hole width follows a single power law up to 10 K. Although the characteristic features of thermal line broadening are similar for the two frequencies, there are clearly discernible differences as well : The respective exponents are slightly different (1.38 vs 1.48 +/- 0.05), and the zero-temperature extrapolated hole widths differ drastically (12 vs 18 +/- 0.5 MHz). From the experiments we conclude that the broadening mechanism is most probably spectral diffusion and that the major contribution must stem from the protein itself.