This investigation of the substitution of lanthanides for mercury in the HgBa2CuO4+delta structure has shown that "1201" cuprates Hg(1-x)Ln(x)Ba(2)CuO(4+delta) can be synthesized for Ln = Ce, Pr, Nd with x approximate to 0.15; whereas for Ln = Sm, Eu, Gd, lanthanide does not enter into the matrix. The XRD and HREM studies confirm the "1201"-type structure and show the statistical distribution of mercury and lanthanides within the mixed [Hg(1-x)Ln(x)0(8)] layer. In contrast to the homologous strontium phases Hg(1-x)Ln(x)Sr(2)CuO(4+delta,) these barium cuprates exhibit superconducting properties, with T-c ranging from 55 K to 83 K when as-synthesized. Annealing in a reducing atmosphere increases T-c significantly, showing that the as-synthesized samples are all overdoped. The most important feature deals with the fact that the critical temperature and the narrow character of the transition for the annealed samples are improved as the size of the lanthanide cation increases, the best characteristics being obtained for the cerium phase that exhibits a T-c of 90 K.