AbstractThe “hardness” components in sea water can be considerably reduced by means of a new economical fixed bed technique called “Reciprocating Flow Ion Exchange”, when used in conjunction with a closed cycle exchanger‐evaporator system.Removal of 87.9% of the calcium and 54.5% of the magnesium in sea water has been obtained with an overall flow rate of 5.0 U. S. g. p. m./sq. ft. Results from bench scale and a 5,000 g. p. d. pilot plant were identical. The virtually continuous cyclic steady‐state process requires a small amount of exchanger (0.5 to 2% of the usual ion exchange methods, such as conventional fixed beds or fluidized beds).The effects of variation in operating parameters ar