New phases of general formula Hg 2 Ba 2 LnCu 2 O 8؊ with Ln؍ Nd-Gd, Dy-Lu were synthesized by high-pressure, high-temperature technique. All the compounds crystallize in space group I 4/m m m and the lattice parameters were found to increase with the ionic radii, ranging from a)3(0648.3؍ A s , c)3(358.82؍ A s for Lu up to a)3(1209.3؍ A s , c)3(689.82؍ A s for Nd. Hg-2212 phases were not obtained with La, due to the high ionic radius of La 3؉ , or with cations having possible valences higher than (؉3), such as Tb, Pr, and Ce. Energy dispersive spectroscopy (EDS) analysis performed on samples with Lu, Yb, Er, and Gd showed significant Hg-deficiency as well as rare earth overstoichiometry. This allows us to suggest that lanthanide cations partially substitute for Hg and that these phases are better described as (Hg, Ln) 2 Ba 2 LnCu 2 O 8؊ . The substitution rate depends on the ionic radii of the lanthanide cation, varying between 25% for Lu (0.977 A s ) or Er (1.004 A s ) to 15% for Gd (1.053 A s ). In the case of Nd-sample it was found that Nd does not replace Hg but Ba instead. The large ionic radius of Nd (1.109 A s ) can probably account for this effect and this phase would be described as Hg 2 (Ba,Nd) 2 NdCu 2 O 8؊ . The susceptibility of Sm, Eu, Dy, and Ho samples exhibits a sharp cusp at a temperature T L between 25 and 28 K. These temperatures are attributed to the anti-