The dependences of the refractive indices of aqueous solutions of dextran (D), poly(ethylene glycol) (PEG), and poly(vinyl pyrrolidone) (PVP), and D-PEG and D-PVP mixtures, on the polymer concentration has been investigated in order to study structural and conformational changes as a function of the increasing polymer concentration. It is shown that in contrast to solutions of single polymers, conformational changes are not observed in aqueous solutions of mixed polymers, even at high concentrations, and the phase separation which is observed is due to a change of structure or state of the water. We have determined the rates of the processes of the phase separation in these aqueous biphasic polymer systems.