Field-cycling NMR relaxometry has been employed to measure the spectral density for double proton transfer in the hydrogen bonds of benzoic acid. Data have been recorded in the intermediate temperature range characterising the crossover region between quantum and pseudo-classical regimes. The quantum-to-classical transition is shown to be smooth and at all temperatures the proton transfer is characterised by a single correlation time. A critical appraisal is given of competing theories. (C) 2004 Elsevier B.V.. All rights reserved.