Precise X-ray powder diffraction patterns of two isostructural triborates, CsB3O5(CBO) and TlB3O5(TBO), have been collected on a D5000 diffractometer with a primary monochromated beam (λ CuKα1=1.5406 Å). Refinement of indexed reflections in the space group P212121 led to: a=6.201(1) Å, b=8.514(2) Å, c=9.176(2) Å, Z=4, Dx=3.363 for CBO and a=5.2156(4) Å, b=8.2659(6) Å, c=10.2240(9) Å, Z=4, Dx=4.773 for TBO. The Smith–Snyder figures of merit are F30=53.0 (0.0101, 56) for CBO and F30=112.9 (0.0074, 36) for TBO. These values are much better than the previous ones published in Powder Diffraction File.
