This contribution addresses apparent negative fouling resistances for crystallization fouling. Two effects contribute to this phenomenon; surface roughness enhances heat transfer in the roughness controlled phase. In the crystal growth phase, surface roughness as well as the constriction of flow cross section due to the fouling layer build-up is taken into consideration. Fouling experiments were carried out in a double pipe heat exchanger with a supersaturated aqueous CaSO4 solution at a Reynolds number of 17,500 corresponding to a flow velocity of 0.65 m s(-1). The measured pressure drop between inlet and outlet allowed the calculation of the integral friction factor for the current surface roughness. With the given friction factor it was possible to estimate the actual heat transfer coefficient of the inner tube. Accounting for the increase in heat transfer caused by surface roughness in the roughness controlled phase, the fouling resistance was recalculated. In the subsequent growth phase flow acceleration due to constriction effects is considered in addition to the roughness effect. Overall the integral fouling resistance and consequently the deposit thickness are underestimated by a factor of up to 2.5 when simply using heat balance. With the proposed approach apparent negative fouling resistances can be eliminated quantitatively. (C) 2010 Elsevier Ltd. All rights reserved.