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Sodium, calcium, sulfur and chlorine in the flue gas of coal-fired boilers are the main cause of ash deposition/fouling in heat exchangers. However, there are still many debates on the mechanism of ash deposition/fouling due to the lack of direct micro evidence. This paper aims to clarify the initial deposition/fouling characteristics of a high-alkali coal by computer-controlled scanning electron microscopy (CC-SEM). The results demonstrate that temperature has an influence on the deposition/fouling compounds. The crystalline compounds in the samples at 900 °C were mainly CaSO 4 and Ca 2 Al 2 SiO 7 , while the compounds in the samples at 500 °C were mainly NaCl. Sulfur had a significant adsorption effect on sodium and calcium at 900 °C, while chlorine had a significant adsorption effect on sodium at 500 °C. A 3-layer microstructure of deposition/fouling was found after the samples were stripped by molecular force. Additionally, deposition/fouling could be divided into three stages: the formation of sodium salt precursors, the growth of loose structural aluminosilicates and the adhesion of particulates on the surface. Sulfates, especially Na 2 SO 4 and CaSO 4 formed a precursor for the initial deposition/fouling of high-alkali coal. • The initial deposition/fouling characteristics of a high-alkali coal were studied. • Samples were stripped by molecular force. • A 3-layer structure of deposition/fouling was observed. • The mechanism of the influence of sulfur and chlorine on deposition/fouling was proposed. • A series of detailed microscopic maps of the deposition/fouling process was obtained. [ABSTRACT FROM AUTHOR]