The study covers the fundamental relationships between the nature of aprotic polar solvents [N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO)] used for casting solution preparation and the structure and performance of poly(amide-imide) (PAI) ultrafiltration membranes. It was found that the distinctive feature of the ternary systems PAI-poly(ethylene glycol) (PEG-400, M-n = 400 g mol(-1)) solvent is gel formation upon the increase in PEG-400 concentration in the solution. The PEG-400 addition to the casting solution was shown to increase pure water flux of the PAI membranes prepared from the casting solutions in different solvents. However, different patterns of the change of pure water flux when gel point is exceeded in PAI-PEG-400-solvent system were revealed. The PAI membranes obtained from the ternary systems PAI-PEG-400-solvent were found to combine high pure water flux at P = 0.1 MPa (130 L m(-2) h(-1) for DMSO, 120 L m(-2) h(-1) for DMA and 70 L m(-2) h(-1) for NMP) and high poly(vinylpyrrolidone) (PVP) rejection coefficients: 98-99% for PVP with M-n = 40000 g mol(-1) and 86-95% for PVP with M-n = 10000 g mol(-1). SEM studies reveal the significant difference in the PAI membrane structure when different solvents are used for casting solution preparation. The structure of the membranes obtained from 20 wt% PAI solutions in NMP and DMA consists of the thin selective layer supported by substructure pierced by elongated macrovoids. Meanwhile, when DMSO and DMF are used as solvents the addition of PEG-400 yields the suppression of macrovoid formation and anisotropic sponge-like membrane structure is obtained.