Hg based 1223 and 1234 phases were prepared by a high-pressure high-temperature technique using precursors obtained by the decomposition of BaO2, CaCO3 and CuO in oxygen flow at 925°C. These phases were found to exhibit much lower Tc's (64 K for 1223 phase, and no superconductivity for the 1234 phase) than the corresponding Hg based samples prepared by different precursors and this even after heat treatments in oxygen flow. The structural refinement of this 1223 phase carried out by using synchrotron X-ray powder data revealed a large deficiency on the Hg site; the occupancy factor for this site was found to be 0.69(1). This may be due to the replacement of some of the Hg cations by C. By IR spectroscopy the characteristic frequencies corresponding to the vibrations of the CO3-2 group were found in these samples. Large amounts of intergrowth defects were defected by electron microscopy and no superstructure spots were found in the diffraction patterns. The replacement of Hg by C in the structures of the Hg based superconductors induces a decrease of the concentration of the extra oxygen in the (Hg, C) layer bonded to the Hg atoms. Since it is this oxygen which oxidizes the (CuO2) layers, its decrease causes a reduction of these layers, which is responsible for the decrease of the superconducting transition temperatures.