The in situ ethanol pre-treatment of commercially available polysulfone (PSU) ultrafiltration (UF) membranes resulted in a 3-fold increase in the pure water flux (PWF) values achieved. Techniques that lead to an increase in flux are of both academic and commercial interest. It is postulated that the mechanisms for performance improvement can be attributed to swelling of membrane skin-layers, as demonstrated by changes in thickness measurements, and consideration of polymer solubility parameters, giving a degree of polymer plasticisation. The modification is accompanied by a hydrophobicity increase – this parameter is linked to a greater fouling tendency. Increases in hydrophobicity contrast with the usual effect of ethanol contact, by enhancing the removal of membrane preservatives and polyvinylpyrrolidone (PVP); a common pore-forming agent. Mechanical property changes were not readily detected, whilst the apparently unaltered sub-layer masked more subtle changes occurring within the dense skin-layer. Directing analysis specifically at the skin layer using colloidal AFM probes allowed a decoupling of changes against the support, showing that the elastic modulus was reduced as a consequence of PVP removal and plasticisation. Moreover, regional elasticity probing allowed observation of spatial inhomogeneities in elasticity; occurring due to the removal of the previously unevenly distributed PVP and leading to pitting. Consequently, the effects of pre-treatment with ethanol are shown to offer advantages by maximising the performance of commercial membranes, though such methods must be used with caution. Elasticity changes that occur may be detrimental to performance if carried out at high transmembrane pressures, where compaction could be assisted.