The diffusion process in solutions of polystyrene (M(w)= 2.74 x 10(5)) in cyclohexane of noncritical composition slows down upon approaching the Liquid/liquid coexistence curve from the homogeneous single fluid phase region of the phase diagram at constant temperatures below the critical temperature and along the Liquid/liquid coexistence curve approaching T-c. This effect reflects the influence of the thermodynamic factor (partial derivative mu>/partial derivative Phi)(T,P) (mu, chemical potential of polystyrene; Phi, volume fraction of polystyrene) on the diffusion coefficient D. The experimental data are discussed in terms of the Flory-Huggins theory of polymer solutions. The composition and temperature dependence of the thermodynamic factor calculated from this model leads to D((Phi,T) curves which qualitatively reflect all features of the experimental data. These findings support the assumption that a slowing down of the diffusion process upon approaching the liquid/liquid coexistence curve of binary mixtures with a miscibility gap is a system independent phenomenon.