Fouling of heat transfer surfaces during subcooled flow boiling is a frequent engineering problem in process industries. Nevertheless, only few experimental and theoretical investigations on this subject can be found in the literature. In this investigation the effect of operating parameters on the deposition of calcium sulphate on heat transfer surface during subcooled flow boiling was studied. A large number of experiments were performed to determine the mechanisms controlling deposition. Fluid velocity, surface and bulk temperatures, concentration and ionic strength of calcium sulphate solutions were varied systematically. Finally, a mechanistic model is suggested for prediction of fouling resistances, which is in good agreement with the experimental data. In this model mass transfer effects are considered for those parts of the surface where heat is transferred by forced convective heat transfer and concentration effects for those parts where bubble formation occurs.