The objective of this investigation was to study the mechanisms of mixed salt crystallisation fouling on heat transfer surfaces and to characterise the structure of the deposits formed on the heat transfer surfaces. The effects of various operating parameters such as solution composition and hydrodynamics of the system on crystalisation fouling of mixtures of calcium sulphate and calcium carbonate, which are the most common constituents of scales formed on heat transfer surfaces, were studied experimentally. After clarification of the effects of operating parameters on the deposition process, deposits on the heat transfer Surfaces were collected for analysis. The crystalline samples were analysed using Scanning Electron Microscopy, X-Ray Diffraction and Ion Chromatography techniques. It was found that crystallisation fouling on heat transfer surfaces exhibits fractal geometry. Therefore, the concept of fractal theory was used to quantify the structure of the deposits by introducing a new quantity called the fractal dimension. (c) 2005 Elsevier Ltd. All rights reserved.