Nanostructured polymeric membranes are key tools in biomedical applications, such as hemodialysis and protein separation, in the food industry, and in the process of obtaining drinking water supplies from seawater. Despite the success in different separation processes, membrane manufacture itself is at risk since the most used solvents are about to be banned in many countries due to environmental and health issues. We propose the preparation of polyethersulfone membranes based on dissolution in the ionic liquid 1-ethyl-3-methylimidazolium dimethylphosphate ([EMIM]DEP). We obtained a series of membranes tailored for the separation of solutes with molecular weights of 30, 5, 1.3, and 1.25 kg mol−1 with respective water permeances of 140, 65, 30, and 20 L m−2 h−1 bar−1. We demonstrate their superior efficiency in the separation of complex mixtures of peptides with molecular weights in the range of 800 to 3500 g mol−1. Furthermore, the thermodynamics and kinetics of phase separation leading to pore formation in the membranes were investigated. The rheology of the solutions and the morphology of the prepared membranes were examined and compared to those of polyethersulfone in organic solvents currently used for membrane manufacture.