Progressive decline in performance of an RO plant, after a period of smooth operation, is described. Research for the reasons and remedy included: 1. Correlation of the recorded decline in performance to problems in feed pretreatment. 2. Selection of elements for individual testing. 3. Elements＇ performance together with their outer features suggested the conduction of element autopsy. 4. Chemical analysis of the fouling film, Cell Test of the sheet membrane, as well as surface analysis by SEM and X-ray: Results revealed that the membranes suffered from accumulation of an iron-based fouling film. Scale deposition took place due to exceeding the system design parameters together with defective pretreatment. Instead of cleaning, the system was forced by increase of pressure so as to maintain the original product rate. This led to element telescoping and localized damage of the thin film surface. Brief discussion of the adequacy of the techniques used is given in view of the characteristics of membrane fouling films.