The main purpose of this investigation was to study the mechanisms of mixed salt crystallisation fouling on heat transfer surfaces during convective heat transfer and sub-cooled flow boiling. To date, no investigations on the effects of operating parameters on the deposition of mixtures of calcium sulphate and calcium carbonate, which are the most common constituents of scales formed on heat transfer surfaces, have been reported. In this work, a substantial number of experiments have been performed to determine the mechanisms controlling deposition. Fluid velocity, heat flux, surface and bulk temperatures and concentration of the solutions were varied systematically. Fractal analysis performed on scanning electron microscopy photographs of the deposits has been used to quantify deposit characteristics. The results of these experiments will be used to develop a mechanistic model for prediction of crystallisation fouling rates of mixed salts.