When burning solid fuels in power plants deposits derived from fuel ashes can be formed and deposited over the heat exchanger surfaces (membrane walls, superheaters, reheaters, etc.) and can lead to reduction of heat exchange, boiler damage and plant availability due to the frequent maintenance and unscheduled shutdowns required to remove the deposits. The mineral matter in the coal, which may be transformed to molten phases during the combustion process, is responsible for these deposits. In a thermal power plant (730 MWth) the combustion behavior of three bituminous coals was investigated in respect to ash and deposit formations as well as the chemical characterization of the deposits. The emphasis is laid on the investigation of the formation of initial and outer deposit layers. Therefore, two types of probes were used to investigate the growth and development of deposits. A temperature-controlled probe is used to investigate the initial layer of the deposits while an uncooled probe is used to investigate the outer layer. The results show that the initial layers of the investigated three coals are enriched in iron likely derived from pyrite in the coals, while the outer deposit layers have a composition similar to that of the bulk fly ash. In addition to the experimental work, equilibrium calculations were performed using FactSage simulation software based on the mineral phases in the coal (pyrite, illite, kaolinite and quartz) and particle analysis 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.