Quasi elastic light scattering is used to investigate the polyelectrolyte and added salt concentration dependence of the diffusion coefficient of sodium polystyrene sulfonate (NaPSS). Two relaxation times are observed in semi dilute regime at low salt concentration. With polyvalent salts (LaCl3and Th (NO3)4) a precipitation of NaPSS is obtained at constant Cm/Cs(Cmand Csare NaPSS and salt concentrations in mono M). The phase separation usually called ‘‘salting out phenomena’’ was studied and some investigations are made in single homogeneous phase.The dynamics show some interesting behavior of the effective diffusion coefficient as a function of NaPSS and LaCl3: The presence of slow and fast modes at low salt concentration and the existence of a large splitting of the relaxation times. a q2dependence of the fast diffusion coefficient and a q3dependence of the slow mode. a power law Dfast≊Cm+0.65at constant salt concentration.