Polymer precipitation curves for the ternary systems PPES/NMP/additive were determined by a titration method. Membrane-forming additives used were H2O,MeOH,EE,EGME and BO．The position of the precipitation curve in the triangle phase diagram reflects the effect of the type of nonsolvent additive on the ternary systems．In addition,the effect can also be demonstrated by the different dropping rate of the intrinsic viscosities caused by various nonsolvent additives．These results indicate that the polymer conformation in solutions varies with the use of different additives．Besides the type of nonsolvent additive,the amount of nonsolvent additive can alter the polymer conformation in solution somewhere．Along the path leading from the initial starting point up to near the precipitation point,a stage would be reached in which the NMP／nonsolvent mixture becomes a theta-solvent for the polymer,and the macromolecules would assume an unperturbed conformation. NMP／nonsolvent theta-compositions were determined by the method of Elias using precipitation point data．A series of ultrafiltration membranes were made from poly(phthalazinone ether sulfone) with NMP as solvent. The membrane structure changes with the increase of the content of nonsolvent additive．When the content of EGME additive is close to 40 wt％ — the theta-composition of NMP／EGME mixture,the structure and properties of membranes change greatly.