The transformation of separate crystals of CaS04·2H20 (DH) into CaS04·0.5H20 (HR) in an aqueaus solution of H3P04 and H2S04 at 90-97°C was studied using a JSM-35CF SEM. The source DR and acids were reagent or eommereial (taken from the produetion of H3P04 by a wet process). DR was allowed to eonvert into HR and periodie samples of the suspension were drawn, filtered, and investigated in the solid phase by SEM. It was shown that the dehydration of reagent DH speeimens oeeurred through the reerystallization mechanism, whereas the transformation of eommereial DR into HH was basieally topoehemieal and developed by a relay race meehanism without any phase transformation front. The observed differenee between eommereial and reagent DR speeimens may be explained by differenees in their erystalline strueture, initial partiele size and by the presenee of impurities in DH and aeids. It is established that ehemieal impurities ean ehange the meehanism of the phase transformation of the erystals DH into HH.