The superconducting compound HgBa2CuO4+x and its precursor Ba2CuO3+y have been studied in an oxygen atmosphere by TG-DTA in the temperature interval 20-1100 degrees C. It was supposed that the succession of chemical processes taking place during the heating (20 degrees min(-1), 1 atm O-2) of HgBa2Cu1.05O4.12 is as follows : [GRAPHICS] It was supposed that the Ba2CuO3.23 compound during heating(1 degrees min(-1), 0.21 atm O-2) and cooling (20 degrees min(-1), 0.21 atm O-2) undergoes following stages : [GRAPHICS]