Herein, the effect of solvent type and polymer concentration on the thermodynamic and viscoelastic properties, and performance of polyethersulfone (PES) ultrafiltration (UF) membranes are investigated. The morphology and mechanical properties of the membranes are examined too. Rheological measurements indicated that solidification and instantaneous demixing occurred fast for PES/N-methyl-2-pyrrolidone (NMP) dope solution. This resulted in a dense skin layer with a finger-like structure confirmed by the scanning electron microscopy images. In comparison, PES/dimethylformamide (DMF) exhibited a delayed solidification and demixing with a sponge-like structure. Rheological characteristics and ternary phase diagrams of two systems are examined to gain insights and make correlations with the morphology of resultant membranes. Performance analysis revealed that membranes derived from PES/NMP system exhibited both improved pure water flux and bovine serum albumin rejection suggesting the superiority of NMP compared to DMF as the solvent of choice for the preparation of PES UF membranes.