A little known compound of the type Mg2KH(PO4)(2)center dot 15H(2)O which is a rare example of highly hydrated salt containing H(PO4)(2) symmetric dimers has been studied with a particular focus on its spectroscopic and thermal features. The infrared spectra have been recorded at room and liquid nitrogen temperatures and assignment of the observed vibrational bands has been proposed. The thermal behavior of Mg2KH(PO4)(2)center dot 15H(2)O has been examined by simultaneous TG/DTA/mass spectrometry technique in a broad temperature interval up to 1000 degrees C New insights on the processes of thermal decomposition as well as on the solid state transformations up to 1000 degrees C have been obtained by combination of the TG/DTA/MS analyses with IR spectroscopic and XRD studies. (C) 2015 Elsevier B.V. All rights reserved.