This paper aims to provide experimental heat transfer results for boiling of CaSO4 solutions on the outside of tube bundles, and also to shed some light into the mechanisms which influence heat transfer fouling under pool boiling conditions. Heat transfer coefficients for three heater rods with an identical diameter of 10.67 mm on a 35 mm vertical pitch have been measured. The independent variables bulk concentration and heat flux have been varied from 0.8 to 1.6 g/L and 33 to 300 kW/m(2), respectively. The experimental results show that the mechanisms of fouling on the middle and top heater substantially differ from those at the bottom heater, due to the dominant effects of bubble impingement on the heat transfer surfaces. In particular, the competing effects on the degree of wall superheat and the supersaturation of the boiling liquid play a significant role. Spalling effects are more pronounced on the upper heaters where thin, friable deposits were found with low density and adherence, particularly at higher heat fluxes and lower concentrations. In terms of tube bundle efficiency, the best results were found for lower concentrations. (C) 2004 Elsevier Ltd. All rights reserved.