This is an optimization study to determine the feasibility of uprating the three MSF distillers 4-6 at Umm Al Nar East, Abu Dhabi, UAE, from 7.2 to 8.5 mgd. The capacity is uprated while maximizing the gained output ratio (GOR) subject to all design and operational constraints. Charts and tables were produced for the operation engineers to vary the plant capacity to any desired value between 10.2 and 6.4 mgd. In addition, the expected performance characteristics of the uprated plants are tabulated. The main variables that contribute to the increase of plant capacity are the top brine temperature and the recycle stream flow rate. The type of polymeric antiscalant used, proper maintenance and the application of a sponge ball cleaning system besides the proper construction materials make it possible to uprate the existing MSF plants within their over-design capabilities. A simple model was used in this study. Predictions for this model were validated and compared with their counterparts from a detailed model. Results from the two models were fairly close. However, those of the detailed model are more dependable to ensure the production of the targeted plant capacity.