In this study, we have reinvestigated the thermal decomposition kinetics of iron(III) phosphate dihydrate FePO4∙2H2O in air atmosphere by TG/DTG and DTA techniques using non-isothermal experiments. The apparent activation energy Eα was determined by the differential and integral isoconversional methods and suggested that the decomposition reaction is complex and follows multi-step processes. A mathematical deconvolution technique using Fraser–Suzuki equation was applied to the DTG curves and allowed the separation of two distinct processes. The apparent activation energies determined by the isoconversional Friedman’s method were 93.05±3.80kJmol−1 and 73.41±3.14kJmol−1 for the first and second process respectively. Using Malek’s procedure for each process, the characteristics of y(α) and z(α) functions showed that the kinetic reaction follows the Johnson–Mehl–Avrami model (JMA(n)). One-dimensional nucleation and growth mechanism occurred firstly with f(α 1)=1.272(1− α 1)[−ln(1− α 1)](1−1/1.272) and pre-exponential factor A 1 =9.11×1010 min−1. After 27% of total conversion, a two-dimensional nucleation and growth mechanism becomes predominant with f(α 2)=2.306(1− α 2)[−ln(1− α 2)](1−1/2.306) and A 2 =1.28×108 min−1. It was concluded that the two decomposition processes of FePO4∙2H2O are closely interrelated and thus neglected the first process leads to incoherent results.