The solubility(s) of the three phases of CaSO4, namely, CaSO4 center dot 2H(2)O (DH), CaSO4 center dot 0.5H(2)O (alpha-HH), and CaSO4 (AH II), in 0.0-18.0 wt % KCl solutions were systemically investigated at temperatures ranging from 85 to 100 degrees C. At fixed temperature, the solubility(s) of the three phases all change with KCl concentration and possess a maximum value. The relative magnitudes of the variance of solubility for AH and alpha-HH are larger than that for DH. This was considered to be correlated to the combined effects of the temperature and concentration of KCl solution on the activity coefficients and water activity. The phase transition behaviors of alpha-HH and DH are presented with possible intermediate phases, which can be well-explained by the solubility difference of the three forms of CaSO4 and the tendency of forming gorgeyite (K2Ca5(SO4)(6)center dot H2O).