HgBa₂CaCu₂O_{6+ delta} has been synthesized through a two-step route, via a precursor but under controlled Hg vapour pressure during the second step. A calculated excess quantity of HgO was put along the precursor, whereon the latter reacted with a stoichiometric amount of the former. Annealing raised the superconducting critical temperature from 97 to 127 K, according to susceptibility measurements. Correspondingly, the lattice parameters also increased. Microwave non-resonant absorption experiments indicated a minute presence of Hg-1223. The thermal stability of Hg-1212 was studied with the help of thermogravity, differential thermal analysis, mass spectroscopy, and X-ray diffractometry. Decomposition became significant above 600 degrees C, but the out-diffusion followed by evaporation of Hg proceeded more slowly; the enthalpies of these two processes were approximately 43 and 130 kJ mol^-1.