Microstructural variations of amorphous FePO 4 and LiFePO 4 (the latter obtained by chemical lithiation of the former) as a result of the annealing temperature have been studied by Thermogravimetric Analysis (TGA)/Differential Thermal Analysis (DTA), chemical analysis, Brunauer-Emmet-Taylor (BET) and Scanning Electron Microscopy (SEM) techniques. Round-shaped amorphous FePO 4 particles 40-80nm in size are obtained after heating (at 400 o C) amorphous FePO 4 .2H 2 O in air (previously prepared by a precipitation route). On further heating at 650 o C, in air, crystalline trigonal FePO 4 of crystallite size <200nm is formed. Round-shaped amorphous LiFePO 4 particles 40-80nm in size crystallize by heating at 550 o C in Ar+5%H 2 for 3h. After thermal treatment, LiFePO 4 particles are interconnected by necks, which resembled a sintering process. The particle size of LiFePO 4 increases with an increase of temperature up to 750 o C, but an abnormal growth is evident at annealing temperatures above 650 o C. DTA analysis showed two exothermic peaks at 547 and 768 o C for FePO 4 due to phase transitions, whereas for LiFePO 4 two exothermic effects at 496 and 567 o C are shown.