Cenospheres are one of the most desired byproducts of coal combustion process nowadays. They are small hollow spheres with roughly 10-1000 mu m in diameter and constitute about 1-2% of the fly ash obtained from the coal combustion processes. Because of their specific properties, namely their low density and high mechanical strength, cenospheres are an important subject of coal fired power plants. In this study, several fly ash samples from a coal-fired power plant located in Poland were analyzed in terms of fly ash composition, cenosphere content and its relation with glass formation principles and combustion conditions. The cenosphere samples, with different size, color and shape, were characterized, chemically and structurally, in order to establish the conditions that favor their formation. For this purpose, cenospheres have been collected and characterized by techniques such as SEM, EDS, XRD, XRF and Raman spectroscopy. Results indicate that cenospheres from coal combustion are constituted by aluminosilicate glasses with some crystalline phases like quartz, mullite and calcite. The high alumina content - roughly 25-27 wt.% - is responsible for the high mechanical strength, while density of most cenospheres is lower than 1 g/cm(3). Regarding the formation process, there are correlations between the amount of cenospheres and the sodium and calcium content, in the different fly ash samples. (c) 2016 Elsevier Ltd. All rights reserved.