This work deals with the influence of CaC03 and Ti02 particles on the precipitation fouling of CaC03 inside a plate heat exchanger. In the absence of particles, a threshold supersaturation ratio Sc must be exceeded in order to have an appreciable amount of deposits. However, the presence of a small concentration of CaC03 particles (40 mg/L) has a synergistic effect, substantially enhancing the deposited mass as well as inducing the onset of scaling at relatively low S values, at which no precipitation fouling is observed otherwise. The enhancement decreases with increasing velocity. Despite the comparable particle size and concentration of TiO2 added to the system, its effect on the precipitation characteristics is insignificant.