Fouling and slagging depend upon the boiler and firing system design, operational parameters and fuel properties. Release of inorganic compounds during combustion and their transformation into critical gaseous species, aerosols and ash particles may substantially affect the boiler efficiency and availability, due to the formation of fireside deposits. The composition of the mineral matter in three bituminous coals and the associated deposits formed at the inlet of the superheater level in a 730 MWth pulverized fuel boiler are discussed in this paper. A cooled probe was used to investigate the initial layer of the deposits while an uncooled probe was used to investigate the outer layer. In all initial layers, spherical (formerly molten) iron-rich particles, likely derived from pyrite were observed. Moreover, particles in the initial layers and mineral phases of the coal samples were determined quantitatively. Based on these analyses thermo-chemical equilibrium calculations were performed using the FACT-Sage simulation software to identify the effect of reducing and oxidizing flue gas atmospheres on the ash melting behavior and to assess their importance on the build-up of the initial deposit layers. The results of this work indicate that the deposition of an ash particle is strongly influenced by the particle's history. Besides its composition, in particular, temperature, and the atmosphere that a particle passes through have a significant influence on the mineral transformation and the adhesion of the particle. (C) 2015 Elsevier B.V. All rights reserved.