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Hydrophilisation of polyethersulfone (PES) based membrane is often achieved by addition of polyvinylpyrrolidone (PVP) leading to a physical blend of the two polymers. This paper shows that the most commonly used membrane for UF in dairy industry is a PES/PVP based one. Nevertheless if hydrophilisation limits the organic fouling, PVP is also the Achilles heel of these membranes. It is particularly true when membranes are exposed to hypochlorite as it is the case for cleaning/disinfection steps. Evidencing the disappearance of PVP from a pristine PES/PVP membrane can be easily achieved by FTIR-ATR analyses. But when one wants to study the ageing of a membrane used in UF it gets more complicated: regardless of the cleaning efficiency the membrane always remains fouled by some proteins. As both PVP and proteins own chemical bounds leading to absorption at the same wavenumber in FTIR-ATR, it thereby prevents the easy highlighting of the PVP degradation. The aim of this paper is to propose a simple treatment of raw FTIR-ATR spectra to dissociate these two contributions, allowing consequently the study of the degradation of a fouled membrane. Then the procedure is applied to a real case study on a spiral membrane. [ABSTRACT FROM AUTHOR]