The thermal decomposition of iron phosphate trihydrate FePO4.3H(2)O was investigated in air using TG-DTG/DTA. The FePO4-3H(2)O decomposes in two steps, and the final decomposition product (FePO4) was studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy. The activation energies of the second dehydration reaction of FePO4-3H(2)O were calculated through the isoconversional methods of Ozawa and Kissinger-Akahira-Sunose (KAS), and the possible conversion functions have been estimated through the Coats-Redfern method. The specificity of thermal decomposition was characterized by identification of the bonds to be selectively activated due to energy absorption at the vibrational level, which was assigned by comparing the calculated wavenumbers with the observed wavenumbers in FTIR spectra. The kinetic model that better describes the second reaction of dehydration for FePO4-3H(2)O is the F,, model as a simple n-order reaction, and the corresponding function is xc4(alpha) = (1-alpha)(2.50) and g(alpha) = -[1-(1-alpha)(-1.50)/(1.50)].