The compounds BaR 2 O 4 , (R = Pr, Tb), where R are two of the less frequent trivalent lanthanides, have been prepared under reducing atmosphere from mixtures of BaO 2 , the R metals, and the oxides Pr 2 O 3 and Tb 4 O 7 , respectively. The crystal structure of BaR 2 O 4 have been refined from X-ray powder diffraction data by the Rietveld method of profile analysis using 31 parameters in each case. Both oxides are orthorhombic, isotypic with SrY 2 O 4 , of the perovskite-related CaFe 2 O 4 structure type, S.G. Pnma (No. 62), Z = 4, a = 10.6113(8), b = 3.6303(3), c = 12.485(1) Å, and V = 480.93(5) Å 3 for R = Pr; and a = 10.4282(8), b = 3.4893(3), c = 12.1809(6) Å, and V = 443.22(3) Å 3 for R = Tb. The R–O distances vary between 2.15(6) to 3.40(6) Å for R = Pr, and between 2.15(3) to 2.91(5) Å for R = Tb.