Whether interconversion of hydrogen bonded centrosymmetric dimers of formic acid occurs through stepwise single-proton or pairwise double-proton transfer is a subject of controversy arising from a misleading treatment of proton dynamics. Quantum entanglement suggests a totally different mechanism. Dimers are in a superposition of indistinguishable tautomers for which interconversion is irrelevant. Disentanglement opens a tunneling channel for single-proton transfer across a nonlocal symmetrical double minimum potential. The nonlocal dynamics of a disentangled dimer is that of half-protons located at four sites with D-2h symmetry. (c) 2005 Elsevier B.V. All rights reserved.