A-site-ordered quadruple perovskites RMn7O12 with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and similar to 1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im (3) over bar structure at similar to 664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at TN1 approximate to 82-87 K at zero magnetic field, but additional magnetic transitions below T-N2 approximate to 12 K were observed in SmMn7O12 and EuMn7O12 at high magnetic fields. Very weak kinklike dielectric anomalies were observed at T-N1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn7O12 shows signs of nonstoichiometry expressed as (Tb1-xMnx)Mn7O12, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn7O12 and EuMn7O12 were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn7O12 and (Tb0.88Mn0.12)Mn7O12 were studied by synchrotron X-ray powder diffraction at 295 K