This work presents studies on the exogenic changes during storage of coal samples. These changes are results of oxidizing and reducing processes. The studied coal samples are of different age and rank The major applied methods are SEM and XRD. Microorganisms were observed among the newly formed mineralization. The neoformed mineral phases are presented by gypsum, K-jarosite, barite, sulphates of Fe, Mg and Na, sulphur, Fe-oxides/hydroxides and framboidal pyrite. Neoformed mineral bacterial colonies were also established, as well as included cell-like bodies in novel mineral matter. It could be supposed that the microorganisms participate in the formation of exogenic mineralization on the mined coals. Observed spontaneous burning of coals appears to be a result of the activity of aerobic microorganisms. (C) 2010 Elsevier B.V. All rights reserved.