Abstract A new superconducting mercury oxycarbonate, Pb 0.7Hg 0.3Sr 4Cu 2CO 3O 7, has been synthesized. This tetragonal phase ( a=3.824 A ̊ , c=16.468 A ̊ ) consists of an intergrowth of two nonsuperconducting compounds, Sr 2CuO 2CO 3 and Pb 0.7Hg 0.3Sr 2CuO 5. It exhibits after optimization a critical temperature of 70 K, with a sharp transition and a superconducting volume fraction of 50%. Its behavior can be compared to that of thallium oxycarbonates previously isolated. This study is completed by a reinvestigation of the 1212 cuprate of the system Hg-Pb-Sr-Ca-Nd-Cu. A superconducting phase with the 1212 structure, similar to that previously obtained but with a significantly different composition, Pb 0.7Hg 0.3Sr 2+ x Ca 0.7Nd 0.3- x Cu 2 O 7, has been obtained, with a T c onset of 100 K. The behavior of the latter is compared with other lead-based 1212 cuprates.