The new quaternary boride series FeRh6-nRun B-3 (n = 1-5) was synthesized by arc melting and characterized by powder and single-crystal X-ray diffraction (XRD), energy-dispersive X-ray analysis, and superconducting quantum interference device magnetometry. Single-crystal structure refinement showed the distribution of the iron atoms in two of three possible crystallographic 4d metal sites in the structure (Th(7)Fe(3)type, space group P6(3)mc). Rietveld refinements of the powder XRD data indicated singlephase synthesis of all the members. A linear decrease of the lattice parameters and the unit cell volume with increasing Ru content was found, indicating Vegard's behavior. Susceptibility measurements show decreasing Curie temperature and magnetic moment (mu(5T)(a)) recorded at 5 T with increasing Ru content from T-C = 295 K and mu(5T)(a) = 3.35 yB (FeRh5RuB3) to T-C = 205 K and mu(5T)(a) = 0.70 mu(B) (FeRhRu5B3). The measured coercivities lie between 1.0 and 2.2 kA/m indicating soft to semihard magnetic materials.