The phase transitions of alpha-calcium sulfate hemihydrate (alpha-HH) in KCl solutions were studied at (80 to 105) degrees C under atmospheric pressure in an attempt to estimate the metastability of alpha-HH and to sketch out the routes for alpha-HH's hydration, dehydration, or recrystallization. The crystal water content and morphology of the product during the reaction period were comprehensively examined. It was found that the metastability of alpha-HH heavily relied on KCl concentration and temperature. The phase transition of alpha-HH is successfully outlined as a function of KCl concentration and temperature. Calcium sulfate dihydrate (DH) is stable in region I and metastable in region III. Anhydrite (AH) is stable in regions III and IV and coexists with goergeyite. HH is metastable in only region II. The results show that alpha-HH tends to hydrate or dehydrate in KCl solution, but there is still a narrow metastable region for alpha-HH crystallization in which KCl concentration and temperature are strictly limited.