In 'classical' modelling approach for crystallisation fouling the deposition rate is taken as a function of concentration driving force multiplied by an Arrhenius type temperature dependent function. Although nucleation sites are known to have a significant impact on crystallisation; this effect is not considered in the 'classical' modelling. This particular approach to define the deposition rate is elaborated and discussed in the current study. The effect of nucleation sites on calcium sulphate crystallisation fouling has been investigated in a plate heat exchanger where the quantity of the nucleation sites is affected by the presence of crystalline particles in process solution and an increase in the amount of deposits forming on heat transfer surfaces. (c) 2007 Elsevier B.V. All rights reserved.