The objective of this paper is to characterise and locate the thin layer of irreversible fouling in a spiral-wound membrane of ultrafiltration of acid whey, after 8000 h of production in a French dairy. Several physico-chemical techniques as SEM-EDX, SEM-FEG FTIR-ATR and AFM were used. Fouling was mainly organic and between 0.5 and 5 mu g of proteins per cm(2) of membrane were reached. The thickness of the fouling layer was higher at the collector axis of permeate and decreased with the distance from this axis on an unwound sheet of the membrane. Ageing of both PES active layer and polypropylene spacers was evidenced by this autopsy. In-depth study was achieved at 50 degrees C for PES membrane ageing in alkaline, acid, alkaline chlorine oxidant and a commercial alkaline cleaning solution (Ultrasil 10). The PES membrane appeared stable in all solutions except those containing chlorine. It was shown by FTIR-ATR that the C-S bond in PES (Ph-SO2-Ph) was broken and led to a Cl-S bond after longterm treatment in alkaline bleach.