Studies of the effect of the number and position of deuterium substituents on the phosphorescence lifetime reveal a significant dependence of the rate of radiationless decay on the positions as well as the number of protons. A theoretical interpretation of the effect of the number and position of deuteration, the intermolecular isotope effect, and the solvent effect on the triplet state lifetimes is given. The experimental results on benzene, naphthalene, and rare earth ions are discussed. For benzene, and naphthalene, the positions of protons of the triplet state are not equivalent and hence the corresponding CH stretching modes are not equivalent. These modes are grouped into two classes; one class is more effective in the electronic relaxation than the other. Under certain conditions the non-radiative constant is linear with respect to the number of modes of each class.