Dynamic light scattering experiments were performed on salt-free aqueous binary mixtures of sodium poly(styrene sulfonate) (NaPSS) with different molecular weights. Three diffusive modes were observed and investigated as a function of the mixture composition, total polymer concentration, and molecular weights of the components of the mixture. In addition to the fast and slow polyelectrolyte modes, which are typical for binary polyelectrolyte solutions, a third new mode, named the "medium mode," was observed. To investigate the role of electrostatic interactions in the mechanism of this mode, similar experiments were performed on analogical binary mixtures of neutral polystyrenes with different molecular weights in benzene. Polystyrene molecular weights were chosen to match the contour lengths of NaPSS samples used. No "medium mode" was observed in this system. Two diffusive modes corresponding to the cooperative diffusion of component 1 and the cooperative diffusion of component 2, respectively, were measured. Experimental data on polystyrene mixtures were successfully fitted by existing theories on the dynamics of neutral binary mixtures. Regarding charged polyelectrolyte mixtures, it was concluded that the medium mode is of electrostatic origin, and its interpretation was discussed qualitatively within possibilities given by current understanding of salt-free polyelectrolytes and theories of light scattering from multicomponent media. (C) 1997 American Institute of Physics.