Dynamics of polymers in mixed solvents are investigated on the basis of linear response theory and mean field arguments. Particular attention is given to the coupling between polymer and fluid fluctuations. This coupling is enhanced by polymer-solvent interaction asymmetry and mixed solvent incompatibility. Cooperative and fluid diffusion constants are analyzed in terms of the interactions in the medium and some predictions for light scattering experiments are made. (C) 2004 Wiley Periodicals, Inc.