The dissociation and recombination of a diatomic molecule in the presence of a third body is considered on the basis of the quantum mechanical scattering theory. The scattering of the third body by the molecule is expressed in terms of the scattering of the third body by the constituent atoms of the molecule, both for the Born approximation and for the impulse approximation. The rate of recombination to a particular molecular level is given by an integral of the differential cross-section for the scattering of the third body by the constituent atoms of the molecule. By integrating over the bound states of the molecule, an approximate expression is obtained for the recombination rate to all levels. The recombination rate to all levels is expressed in terms of the rate of diffusion of the third bodies in a gas of the dissociated atoms.