The stability of Hg 1 - x Ba 2 Ca n - 1 Cu n O 2 n + 2 + δ [Hgl2(n-1)n] and its competing phase, HgCaO 2 , at synthesis temperatures were studied in terms of their equilibrium Hg vapor pressure P H g . It was found that the stability of Hg12(n-1)n varies with n in a complex way. It was also observed that traces of moisture and/or carbon dioxide reduce the P H g of HgCaO 2 , while partially substituting Re, Pb and Mo for Hg suppresses the P H g of Hg12(n- 1)n. Possible relationships between the beneficial substitution effect, the change of cation valence, and the defect structures in Hg12(n-1)n are proposed.