Today, the reverse osmosis (RO) becomes a process of first importance for fresh water production worldwide. For this reason, downtimes due to repairing operations (following breakdowns, blockage of membrane, pressure losses, etc.) or preventive maintenance (cleaning of membranes, changes in components, etc.) have to be minimized in duration and frequency to ensure maximum availability. Indeed, improving the availability (or the reliability) of the RO plant as a whole system, objectively leads to a substantial reduction in operating costs and maintenance. In this paper we consider two methods for availability and effectiveness assessment of an RO unit, where the main objective is economic optimization. The two methods are Reliability Block Diagram Method (RBD) and Fault Tree Analysis Method (FTA). For the validation of both methods, we used data related to an RO unit operating in Kuwait and corresponding to four years operation. A comparison between both methods was performed in order to highlight the efficiency and the limitation of each one. It was shown that all the subsystems (pretreatment, dosing, etc.) have a good availability. Slightly lower availability was determined for the high pressure pump. A sensitivity study was conducted to determine the critical components for the availability of the RO plant. The obtained results show a high influence of the availability of the high pressure pump on the total availability of the system. Hence, particular attention should be taken on the selection and the maintenance of the high pressure pump. The comparison between the two methods RBD and FTA shows that the first method is more appropriate for the availability assessment because it allows accurate modeling of different complex configurations. (C) 2012 Elsevier B.V. All rights reserved.