At the Thermal Power Plant in Oroszlány (Hungary), a huge amount of by-products have been produced since 1961. In this survey, coal and other by-products were examined (fly ash, bottom ash, fluidised bed bottom ash, gypsum and slurry-type ash). The natural isotopes were determined using an HPGe detector. It was found that the radionuclide content of coal was significantly lower (226Ra = 45.3±6.3, 232Th = 26.3±5.7 and 40K = 210±21 Bq kg−1) than that in the case of ashes other than the fluidised type. The average values of the bottom ash—deposited in the largest quantities—were three times higher than those for coal (226Ra = 144±18,232Th = 84.3±14 and 40K = 260±25 Bq kg−1). In the case of fractionised bottom ash, the radionuclide content of <0.1 mm was 45 % higher than that of >1.6 mm and the massic radon exhalation of <0.1 mm was approximately four times higher than that above this range.