Dynamic light scattering experiments were performed on salt-free aqueous binary mixtures of sodium poly(styrene sulfonate) 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. The fast and slow polyelectrolyte modes, which are typical for binary polyelectrolyte solutions, are observed also in binary mixtures and maintain their basic characteristics. The fast diffusive mode is independent of the total polymer concentration, mixture composition, and molecular weights of the two components, in agreement with results obtained on binary solutions. Only one slow diffusive mode is observed in binary mixtures, in spite of the fact that polyions with appreciably different molecular weights in binary ablutions give slow modes on appreciably different time scales. Therefore, the slow polyelectrolyte mode cannot be identified with any single-chain property. Oppositely, our results confirm "multichain character" of the processes behind the slow mode mechanism. In addition to the fast and slow polyelectrolyte modes, a new mode, named the "medium mode," was observed. The investigation of the medium mode is presented in the subsequent paper in this issue of the journal. (C) 1997 American Institute of Physics.