This paper investigates a methodology for the characterisation and location of a thin layer of irreversible fouling in a spiral-wound ultrafiltration membrane by using several physico-chemical techniques as SEM-EDX, SEM-FEG, FTIR-ATR and AFM. Application to analysis of a spiral-wound PES membrane for UF of skimmed milk, after 4 years of production in a French dairy plant, is detailed. From the analyses, it was evidenced that all parts of the spiral module did not have the same behaviour towards fouling and cleaning. The mapping obtained from SEM-EDX shows that irreversible fouling is located near the collector axis of the permeate and is independent of the inlet and outlet of the membrane. Significant differences are observed between the inner/outer and middle sheets of the spiral membrane. This is probably closely connected with the distribution of the fluid velocity in the spiral membrane. It would be important to compare such analyses with now available computational fluid dynamics (CFD) data related to this complex spiral geometry. Such an approach would lead to improvement of the cleanability of such systems.