It is shown that the two exponential modes in the dynamic scattering intensity obtained by scattering experiments from ternary polymer mixtures cannot, in general, be identified with the interdiffusion and cooperative diffusion processes, contrary to recent suggestions in the literature. Conditions under which such an identification is possible are obtained. General expressions for the interdiffusion and cooperative diffusion coefficients associated with a given pair of components a and b in a ternary, or, in general, multicomponent polymer mixture are obtained in terms of the partial mobilities μαβ and partial scattering functions S αβ(q) of the a and b polymers. In the framework of the random phase approximation, a new expression of the interdiffusion coefficient in an incompressible ternary mixture of a and b homopolymers in a matrix of c homopolymers is found in terms of the tracer diffusion coefficients of the three species. It is shown that the results of the controversional “fast mode” and “slow mode” theories are obtained from this new expression as two limiting cases.