In drinking water production plants, the use of oxidants such as sodium hypochlorite during in-place cleanings may impair the membrane integrity and radically impact the ultrafiltration process efficiency, leading to potential contamination of the permeate water with pathogens. This study investigates the effects of hypochlorite exposure on the properties of a commercially available UF hollow fiber. Mechanical performances and water permeability appeared to be greatly affected by the contact with hypochlorite. Monitoring them olecular changes by X-rayphoto electron spectroscopy (XPS), attenuated total reflectance infraredspectroscopy (ATR-IR), size exclusion chromatography (SEC) and VITA-mode atomic forcemicroscopy (VITA-AFM) revealed high stability of the main polymer constituting the membrane (i.e. polyethersulfone (PES)) and very high reactivity of the additive (i.e. poly (N-vinyl pyrrolidone (PVP)) towards immersion in aqueous sodium hypochlorite solution with maximal reaction rate for neutral to slightly basic pH. Correlation of those results unexpectedly leads to the conclusion that the over all membrane properties changes are governed by the additive fate.