Scale formation of CaCO3 in a plate heat exchanger is investigated in the presence of various types of added particles under isothermal conditions. The parameters examined include the degree of supersaturation, the type of particles, the flow velocity, the particle concentration and the direction of flow inside the heat exchanger. The key result of this work is the strong synergistic effect of fine aragonite particles on the deposition rate and the morphology of the deposits. On the contrary, the presence of fine titanium oxide and of relatively large calcite particles does not seem to affect the main scaling characteristics of CaCO3. (C) 2003 Elsevier Ltd. All rights reserved.