The effect of hydrogen/deuterium exchange on protein hydrogen bond coupling constants h3JNC′ has been investigated in the small globular protein ubiquitin. The couplings across deuterated or protonated hydrogen bonds were measured by a long-range quantitative HA(CACO)NCO experiment. The analysis is combined with a determination of the HN/DN isotope effect on the amide group 1JNC′ couplings and the 15N and 13C′ chemical shifts. On average, H-bond deuteration exchange weakens h3JNC′ and strengthens 1JNC′ couplings. A correlation is found between the size of the 15N isotope shift, the 15N chemical shift, and the h3JNC′ coupling constants. The data are consistent with a reduction of donor-acceptor overlap as expected from the classical Ubbelohde effect and the common understanding that HN/DN exchange leads to a shortening of the N-hydron bond length.