This work presents a comparative study on the formation of polyethersulfone ultrafiltration membranes via nonsolvent-induced phase separation (NIPS) in two different solvent systems. N-methyl-2-pyrrolidone was chosen as conventional solvent and 2-pyrrolidone as a greener alternative. The overall objective was to obtain a mechanistic clarification of the membrane formation process in dependence of the most important controlling parameters. By performing different series of experiments, it was possible to determine the differences between the two solvents regarding the effects of variations in nonsolvent additives, polymer concentration, and precipitation conditions. It was found that a raising concentration of several nonsolvents, the increase of the polymer concentration and changes in the precipitation conditions can suppress the formation of macrovoids, regardless of the applied solvent. In contrast, differences were observed with regard to the performance of the membrane prototypes. This study improves the understanding of membrane formation via NIPS and identifies the effects of different variables. It shows that the choice of the solvent is essential for the dominating formation mechanisms and therefore for the resulting membrane features. It also proves that green solvents can substitute hazardous solvents if the influencing variables are well-understood in order to control them for obtaining desired membrane properties. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48852.