Abstract The vibrational relaxation times of HF and DF have been calculated on the basis of the vibration-rotation-translation energy transfer theory over the temperature range from 400 to 4000°K. Up to about 2500°K, DF molecules always relax slowly compared to HF, but at still higher temperatures they relax faster. This alternation in the isotope effect is due to the increased efficiency of the translational motion in removing the vibrational energy at elevated temperatures in DF-DF. In HF-HF: the maximum value of the relaxation time at 1 atm appears at ≈800°K, and in DF-DF it occurs at ≈700°K. The appearance of the maximum values is due to the dipole-dipole attraction. The calculated values are in reasonable agreement with experiment.