The influence of the concentration and molecular weight of polyethylene glycol (PEG, 40040000 g mol(-1)) on the phase state and viscosity of ternary polysulfone-polyethylene glycol-N,N-dimethylacetamide solutions has been studied. It was shown that an increase in PEG molecular weight (MW) results in a decrease in the region of existence of homogeneous solutions on the phase diagram due to polymer incompatibility, and in an increase in the viscosity of polymer solutions. At a constant PEG concentration (5%) the viscosity depends on PEG MW in a complicated way: in the range of PEG molecular weights 10006000 g mol(-1) the viscosity is nearly unchanged, but when the PEG MW exceeds 6000 g mol(-1) a sharp increase in the viscosity of the polymer solutions is observed. It was shown that changes in the membrane performance are determined by PEG concentration in the dope solution. At a PEG concentration of 5% an increase in PEG MW results in an increase in membrane performance and a decrease in the rejection capability; at an increase in PEG concentration in the dope solution up to 25% the maximum pure water flux was observed for PEG-400. The bubble point test showed that with an increase in PEG molecular weight a fraction of large pores, which can be considered as selective layer defects, increases.