The modification of the habit of CaCO 3 crystals spontaneously deposited from highly supersaturated solutions treated with scale inhibitors is reported. Scanning-electron-microscope investigations reveal the marked effect of the scale-inhibitor on the crystallization and the modification of crystal habit. The method described is suitable for determining the inhibitor efficiency. On the basis of the alterations of the morphology of water-formed deposits the production of removable aggregates could be expected.