Cr₃B₇O_l3Br, M_r = 519.55, cubic, F43c, Z = 8, a(Mo Ka) = 0.71073 ,~, F(000) = 1968. At 298 K: a = 12.1524 (1)/~, V = 1794.68 (3) A³, D_x = 3.845 Mg m^-3, /mu = 8.533 mm^-1, final R = 0.020 for 206 unique reflections with I > 3/sigma(I); shortest interatomic distances (A): Cr--O = 2.061 (2), Cr--Br = 3.03810 (3), B--O = 1.436 (3), O--O = 2.388 (2). At 113K: a=12.1340 (3) A, V=1786.54 (13) A³, D_x, =3.863Mg m^-3, /mu=8.533mm^-1, final R=0.025 for 281 unique reflections with I> 3/sigma(I); shortest interatomic distances (A): Cr--O = 2.057 (2), Cr--Br = 3.0335 (1), B--O = 1.436 (3), O--O = 2.390 (2). The planarity of the metal-centred CrO₄ groups does not differ significantly from that in other chromiumbased cubic boracites and generally increases as the temperature is lowered.