It has been shown previously that collective diffusion as observed by dynamic light scattering from a binary polymer solvent system can be strongly influenced by longitudinal viscoelasticity. The formalism of ref 1 describing such a coupling is applied to a ternary system comprising either two types of homopolymers or one type of diblock copolymer in a solvent. It has to be distinguished between the decay of concentration fluctuations of the entire polymer subsystem, which is related to cooperative diffusion, and the decay of composition fluctuations, which is related to interdiffusion. Both, cooperative diffusion and interdiffusion, are found to couple to viscoelasticity. In addition, the interdiffusion process is affected by the total force exerted between the two species, which may create new effects.