The performance and the cost of three vapor-compression (VC) seawater distillation systems of unit capacities 2 to 10 mgd are analyzed within the current distillation technology and its scale control. The results of the analysis are summarized in a cost-efficiency diagram, increased unit size is achieved at the expense of a tolerable decrease in efficiency. The design features of the desired compressors and the models used for cost-performance analysis are described elsewhere. The trend of the results points to the importance of developing a reliable chemical treatment that controls the scale for boiling seawater at atmospheric pressure. Excluding thr: cost contribution of the treatment to production cost of distillate, the development can reduce the current cost of seawater distillation by as much as 30% while increasing efficiency by about 50%.