Hybridization of seawater reverse osmosis (SWRO), desalting technology and the multi-stage flash (MSF) has been considered to improve the performance of the latter and reduce the cost of desalted water. Coupling of the two processes could be made on different levels of integration and the resulting water cost will depend on the selected configuration: not only the plant configuration that affects water cost but also the cost of materials of construction, equipment, membranes, steam, energy, chemicals, etc. Despite the advantages of hybridization, it is noticed that the present number of hybrid plants is very limited. Many workers have investigated the hybridization problem of RO and MSF technologies from different angles. The present work is a trial to participate in these efforts to throw more light on the feasibility of the hybridization idea through an optimization study for the prediction and comparison of the minimum water cost of seven different designs of hybrid plants. The selected designs comprise brine recycle and once-through MSF units coupled with a single-stage SWRO unit receiving power and steam from an external source. In addition, the study includes a two-stage SWRO desalting plant and a brine recycle MSF plant, each having the same capacity of the other hybrid plants. This study is made up of three parts: (1) modeling, problem definition and algorithms; (2) presentation and discussion of the optimal designs obtained and the associated water costs and other parameters; and (3) sensitivity analysis of the results to variations in cost elements and operating conditions. The first part is presented here where the descriptive and cost models are outlined, the optimization problem is formulated, and the computational loops repeatedly performed to reach the optimal solution for three case studies are flowcharted.