The effect on the vibrational and rotational relaxation time of de‐excitation by photon emission in four gases possessing a permanent electric dipole moment is investigated. Such radiative energy losses are independent of pressure, and may be expected to control the relaxation process at low pressure when collisions become relatively infrequent. Calculations are shown forN2O, OCS, andSO2. Estimates of the efficiency of collisions to deactivate a vibrational level in HCl, indicate that the radiation effect on the vibrational relaxation time of this gas should be the easiest to measure; at a pressure of 10−2–10−3atmos, de‐excitation by radiation should be as important as collisional de‐excitation. Radiative de‐excitation will become increasingly important at low temperatures because of the consequent increase of the collisional relaxation time.