The crystallization of calcium oxalates during displacement was investigated by thermomicroscopy, thermogravimetry, x-ray, and scanning electron microscopy (SEM). Gypsum cleavage plates were immersed in solutions of oxalic acid and alkali oxalates. Whewellite and/or weddellite formed on the gypsum crystal surface. Different amounts and crystal forms were found. An evaluation of the growth rate was possible through thermogravimetry.