A slow cooling method using molten copper as a flux was applied for crystal growth in the systemsR-Rh-B andR-Rh-B-C (R=rare earth element).Ternary borides: Single crystals ofRRh3B2(R=Gd, Dy, Er, Tm, Yb, and Lu),RRh4B4(R=Sm, Dy, Er, and Tm), andRRh3B (R=Sm, Gd, Er, and Tm) were successfully obtained. Crystals of ErRh3B2are hexagonal rectangulars belonging to the monoclinic systema=0.5355(1) nm,b=0.9282(1) nm,c=0.3102(1) nm, andβ=90.89(3)°. A super lattice appears along both theaand thecaxes with the periods of 3aand 6c, respectively. Single crystals of ErRh4B4were obtained as rectangulars with a primitive tetragonal structure:a=0.5292(4) nm andc=0.7379(3) nm. Single crystals of ErRh3B were extracted as cubes. ErRh3B is cubic witha=0.41466(1) nm. PrRh4.8B2crystallized as hexagonal plates. This new compound is orthorhombic:a=0.9697(4) nm,b=0.5577(2) nm, andc=2.564(3) nm.Quaternary borocarbides: Single crystals of the new tetragonal compounds ofRRh2B2C have been obtained forR=Er and Gd as thin plates. Lattice parameters of e.g., ErRh2B2C, area=0.36848(2) nm andc=1.05520(3) nm.