[en] It has long been known that the substitution of deuterium for hydrogen in hydrogen-bonded materials can lead to a change in the geometry of the hydrogen bonds - the so-called Ubbelohde effect. There can also be significant accompanying changes in the physical properties of the material. Among the most striking examples is the increase in the transition temperature, T_c, of hydrogen-ordering systems of the KH₂PO₄ type by >100 K on full deuteration. In all of these systems, T_c decreases under pressure, making it possible in principle to compare protonated and deuterated forms at the same T_c by applying pressure to the latter. Here we report the results of a high-pressure neutron diffraction study of the deuterated H(D)-ordering material PbDPO₄. We find that much, and possibly all, of the increase in T_c on deuteration is attributable to the accompanying changes in the hydrogen-bond dimensions. This suggests that purely mass-dependent effects on the quantum-mechanical tunnelling between the equivalent H(D) potential minima - an explanation that has been favoured previously -have little or no direct influence on T_c. Substantial revisions to the theory of this type of ordering transition are apparently required. (author)