Samples of a fly and a bottom ash, each before and after ignition at 960 degrees C, have been studied by X-ray diffraction, Mossbauer spectroscopy, carbon analysis, and thermogravimetric analysis with the aim to explain an observed negative loss on ignition. Ashes after ignition contain more maghemite resulting from oxidation of newly formed magnetite. Moreover, the fly ash that already contained magnetite exhibited an increase of hematite after ignition. Hercynite present in both ashes transforms to hematite and magnetite after the ignition. All these oxidation processes are responsible for a weight gain which may compensate the loss due to the burning of the remaining carbon in the ashes. Also, alpha-Fe is formed after ignition which may have originated from wustite. (C) 2010 Elsevier Ltd. All rights reserved.