[en] High-resolution neutron-diffraction techniques have been used to determine the crystal structure of deuterated squaric acid, D₂C₄O₄, in its ordered phase at 298 K. Comparison with H₂C₄O₄ at the same temperature reveals the effects of deuteration alone on the crystal structure. The main changes are seen in the geometry of the O-H(D)...O bonds; the overall O...O bondlength increases by ≅ 0.020 A on deuteration, and the O-D bond in D₂C₄O₄ is ≅ 0.015 A shorter than O-H in H₂C₄O₄. These changes probably account for much of the increase in the H(D)-ordering temperature, Tc, in this system, from 371 K in H₂C₄O₄ to 516 K in D₂C₄O₄. (orig.)