Phase diagrams of the polyphenylsulfone (PPSU) – polyethylene glycol (PEG) – N-methyl-2-pyrrolidone (NMP) system have been plotted as a function of PEG molecular weight. For 15 wt % PPSU solutions containing 15 wt % of PEG with molecular weight of 6000 – 40 000 g mol –1 , the occurrence of an upper critical solution temperature (UCST, solution – gel transfer (gel point)) and a lower critical solution temperature (LCST) was found. According to the phase diagrams analysis, a method for the preparation of high flux PPSU membranes using the systems with UCST and LCST in an experimentally attainable temperature range has been proposed. The method involves casting solution processing at a temperature of the existence of a one-phase homogeneous solution (UCST < T < LCST), and the use of a coagulation bath at a temperature between UCST (gel point) and LCST. Membranes prepared from 15 wt % PPSU solutions with the addition of PEG-20000 were found to have the highest pure water flux (500–1000 L m –2 h –1 at 0.1 MPa), and a human serum albumin rejection of ca. 90%.