Abstract This report is a second update of costs, originally presented in ORNL/TM-5070, which gave cost estimates for desalting seawater and brackish water based upon first quarter 1975 costs. The first update was based upon 1977 costs. The specific input to the report includes two earlier U.S. Department of Energy reports, recent work sponsored by the U.S. Office of Water Research & Technology, and new equipment and operating cost input from major equipment suppliers and users. Cost estimates are given for desalting seawater by distillation and reverse osmosis, and for desalting brackish water by reverse osmosis and electrodialysis. The report includes the cost of generating steam and electrical energy on site using coal-fired boilers as well as oil-fired boilers, and dual purpose nuclear/electric seawater distillation plants. The energy costs for both reverse osmosis and electrodialysis are based upon the availability of electricity at a fixed rate. Cost data were computed as a function of plant size, and include both capital costs and construction costs which are considered as typical. These assumptions are used to develop the reference cases of total water cost. The manner of presentation is such, however, that the costs can be easily adjusted to reflect local conditions.