Tschermigite, (NH4)Al(SO4)2 ∙ 12H2O, is a widespread mineral in efflorescence at geothermal fields associated with Kambalny and Koshelevsky volcanoes (Southern Kamchatka, Russia). The determined temperature ranges of stability of tschermigite and products of its high-temperature evolution by in situ powder X-ray diffraction: tschermigite (23-60°C) → X-ray amorphous phase (70-200 °C) → godovikovite (210-390°C) → millosevichite (380->620°C). This implies decomposition of tschermigite (23-60°C) first to the interim X-ray amorphous phase (probably melt) from which godovikovite is then formed, without direct transformation of tschermigite to godovikovite, as it was suggested in some papers previously. Deammonization of godovikovite to millosevichite occurs at 380-390°C. Thermal expansion coefficients are given for tschermigite, godovikovite and millosevichite with discussion on some peculiarities on their formation. Regarding IR bands that refer to ammonium ion: ν
