The objective of this study was to conduct the autopsy of nanofiltration membranes obtained from a full scale filtration. Membrane sheets were obtained from spiral-wound modules taken at different stages of the train of a membrane filtration unit of a drinking water treatment plant. The foulant layer was physically removed from the membrane, sonicated in MilliQ water and centrifuged. The foulant material was separated into two sub-fractions, soluble and insoluble. The two fractions were quantified by weight and characterized by elemental, FT-IR, 13CNMR and pyrolysis GC/MS analyses. The low ash content of the isolates indicated that the organic phase was the major constituent of the foulant material. Calcium, iron and aluminum were the major inorganic species. Elemental analysis of all isolates were quite similar to the one reported for membrane cell residues (i.e. peptidoglycan) isolated from surface waters (higher in H and N as compared to humic substances type material). The structural characterization confirmed the microbial origin of the organic matrix (biofouling). For all selected membranes, the insoluble foulant material was found to be more enriched in nitrogenous structures as compared to the soluble material. The soluble fraction was found to incorporate a larger proportion of carbohydrates (i.e. exopolysaccharides). Includes 9 references, tables, figures.