The phase inversion technique was used in this study to prepare polyethersulfone membranes. After optimizing the synthesis factors, like the relative air humidity, the concentration of the polymer and the used solvent, nanoporous membranes were obtained. Characterization of these membranes indicated that they were similar to commercial polyethersulfone nanofiltration membranes, except for the molecular weight cut-off (MWCO). In spite of the larger MWCO, the self-made membranes showed similar or even higher retentions when filtrating negatively or positively charged compounds, and this in combination with very high permeate fluxes.