A further examination of the mechanism for proton transfer in carboxylic acid dimers is made, in particular the way quantum tunnelling and pseudo-classical dynamics are manifested in spin-lattice relaxation. New low field measurements of the proton T, are presented. In support of the analysis made in the earlier Letter [W. Wu, D.L. Noble, A.J. Horsewill, Chem. Phys. Lett. 402 (2005) 519], it is shown that a single correlation time model characterises the dynamics across the wide range of temperatures studied. This supports the view that the transition between quantum tunnelling and classical hopping is a smooth and continuous one. Some experimental and conceptual implications are discussed. (c) 2006 Elsevier B.V. All rights reserved.