Polymeric membrane flat sheets as well as hollow fibres are widely used in membrane bioreactors in order to separate biologically cleaned water from suspended solids. The polysulfone (PS) flat sheet and polyethersulfone (PES) hollow fibre membranes have been chosen for characterization and to investigate the impact of wetting agents on membrane filtration performance. Virgin PS and PES ultrafiltration membranes were characterized by simple filtration test with deionised water to determine their permeability. It was found that during pure water filtration with hollow fibre PES membranes the permeability was changing when filtration tests were run at permeate fluxes of 15 L/m(2) h, 30 L/m(2) h, 15 L/m(2) h, 50 L/m(2) h and 15 L/m(2) h in subsequent filtration periods. This phenomenon is explained by membrane wettability. Additionally, it has been shown that the permeability of virgin membranes varies in a broad range. The absolute permeability value depends on the degree of pores employment in the filtration and increases with higher transmembrane pressure. In order to wet a maximal number of pores, the membranes were treated with different wetting agents like acetone, isopropyl alcohol and ethanol. The observed changes in permeability demonstrated that the three chosen wetting agents play an important role for the activation of smaller pores through a reduction of the surface tension. Finally, the impact of pore wetting on the filtration of sludge supernatant, dextran solution and Bovine serum albumin (BSA) solution was tested to investigate whether there is an effect on membrane fouling.