The samples of the pseudobinary system (FePO4)(1-x)-(FeVO4)(x) with x = 0-1 were prepared by precipitation from a solution of iron nitrate and ammonium phosphate/vanadate, and the evolution of their structure during the heating of the precipitate within the temperature range of 20-1000 degrees C was studied by thermal analysis, X-ray diffraction and infrared spectroscopy. The samples dried at 120 degrees C were amorphous having a high specific surface area of 160-222 m(2) g(-1) and their crystallization took place at 440-560 degrees C. The regions of solid solutions between the two boundary phases were smaller than 10 mol % of the second phase and a new crystalline phase was formed in the system at the composition of FePO4:FeVO4 = 1:1.