Abstract Ba 2CuO 3/HgBa 2CuO 4+ δ and Mn 3O 4/Mn 2O 3 couples were employed to control the mercury and oxygen partial pressures, respectively, during the synthesis of chromium doped Hg-1201 phase in the sealed quartz tube. Suitably adjusted conditions allowed us to prepare series of single phase samples of the final composition Hg 0.7Cr 0.2Ba 2CuO 4+ δ determined by weight changes and EMA. The oxygen stoichiometry was modified by the oxygen partial pressure in the range of pO 2=0.05–1.59 atm. The X-ray diffraction and magnetic susceptibility study revealed linear dependence of the structural parameters and parabolic one of T c vs. log pO 2. Multiphase samples, however, form for too high oxygen activity, namely for pO 2=5.16 atm.