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Single-phase polycrystalline samples of the compounds MRh6B3 (M = Fe, Co) as well as single crystals of CoRh6B3 have been synthesized by arc-melting the elements under a purified argon atmosphere in a water-cooled copper crucible. The characterization of the new phases was achieved by using single-crystal and powder X-ray diffraction as well as EDX measurements. The two phases are isotypic and crystallize in the hexagonal Th7Fe3 structure type (space group P63mc, no. 186, Z = 2). In this structure, the magnetically active atoms (Fe, Co) are preferentially found on only one of the three available rhodium sites, and together with rhodium they build a three-dimensional network of interconnected (Rh/M)3 triangles. Magnetic properties investigations show that both phases order ferromagnetically below Curie temperatures of 240 K (for FeRh6B3) and 150 K (for CoRh6B3). First-principles DFT calculations correctly reproduce not only the lattice parameters but also the ground state magnetic ordering in the two phases. These calculations also show that the long-range magnetic ordering in both phases occurs via indirect ferromagnetic coupling between the iron atoms mediated by rhodium. This magnetic structural model also predicts the saturation magnetizations to be 4.02 μB for FeRh6B3 (3.60 μB found experimentally) and 2.75 μB for CoRh6B3. Furthermore, both phases are predicted to be metallic conductors as expected for these intermetallic borides. [ABSTRACT FROM AUTHOR]