The thermal decomposition of manganese dihydrogenphosphate dihydrate Mn(H2PO4)(2)center dot 2H(2)O was investigated in air using differential thermal analysis-thermogravimetry. Mn(H2PO4)(2)center dot 2H(2)O decomposes in two steps and the final decomposition product (Mn2P4O12) was studied by X-ray powder diffraction and FT-IR spectroscopy. The activation energies of the dehydration and decomposition steps of Mn(H2PO4)(2) center dot 2H(2)O were calculated through the isoconversional methods of Ozawa and Kissinger-AkahiraSunose and the possible conversion functions have been estimated through the Coats-Redfern method. The activation energy calculated for the dehydration and decomposition of Mn(H2PO4)(2) center dot 2H(2)O by different methods and techniques were found to be consistent. The possible conversion functions of the dehydration and decomposition reactions for Mn(H2PO4)(2)center dot 2H(2)O were "cylindrical symmetry" and "three-dimension diffusion", respectively. Activated complex theory has been applied to each step of the reaction and the thermodynamic functions Delta H*, Delta G* and Delta S* are calculated. These values for two stages showed that they are connected with the introduction of heat and are nonspontaneous processes.