The temperature-dependent vibrational pure dephasing of the CO stretching mode of carbonmonoxyhemoglobin (HbCO) in an ethylene glycol:water mixture is reported and compared to previously measured dephasing of carbonmonoxymyoglobin (MbCO). HbCO displays a T-1.3-dependent pure dephasing rate between 15 and similar to 150 K, suggesting glass-like behavior. Above 150 K, the temperature dependence becomes steeper. The functional form of the HbCO and MbCO pure dephasing temperature dependences are identical within error. However, the hemoglobin pure dephasing is 27% smaller at all temperatures studied, suggesting that the fast dynamics or the protein electric field-CO coupling is smaller in hemoglobin than in myoglobin.