Some properties of ultrafiltration polysulfone membranes modified with n-BuNH2 and n-BuNH2/Ar plasmas were studied. Contact angle measurements, XPS and SEM were used to characterize the surface changes. Pore size estimation, water transport and filtration parameters helped to describe changes in filtration performance. Presence of argon in plasma environment stabilized plasma but made it more aggressive; pore size increased and surface etching took place. This plasma grafted a smaller number of nitrogen features - most of them in the form of amine. Excellent filtration performance was observed for BuNH2/Ar plasma-modified membranes in an acidic environment.