The intermolecular contribution to the relaxation of singlet states has been derived on the basis of a translational-rotational diffusion model that describes molecules as impenetrable spheres which translate and rotate in an isotropic low-viscosity medium. The equations for the relaxation rate constants obtained are discussed and the dependence on physical parameters is exploited. Theoretical predictions are compared with experiments when the intermolecular relaxation is due to both protons and deuterons present in the sample. An agreement between experiments and theory of ±4% was obtained when the physical parameters are estimated from first-principles calculation.