Suspended CaSO(4)center dot 2H(2)O was recrystd. to CaSO(4)center dot 0.5H(2)O (I) in aq. soln. of H(3)PO(4)(24-28% by mass of P(2)O(5), 4-8% by mass of SO(4)(2-)) at 88-92 degrees C for 1-4 h to det. the optimum conditions. The I content in the solid phase increased up to above 90% (conversion degree near to 100%) with the increasing temp., and P(2)O(5) and SO(4)(2-) contents in the soln.