A classical intersecting-state-model of harmonic oscillators was applied to the study of proton-transfer reactions. The activation free-energy barriers were found to be strongly dependent on the bond order of the transition state,n^†, with this parameter ranging between 0.5 and 1.0. The carbon acids are closer to the first limit, HF is near to the second, and the nitrogen and oxygen acids are somewhere between. Such differences can be attributed to the polarity of the XH bonds. For a considerable number of reactions the distance between the minima of the harmonic oscillators is virtually independent of the reaction free energies, ΔG°, but for others such distance increases with an increase in |ΔG°|. The observed increase depends on the mixing entropy, λ. Non-linear Brönsted relationships and anomalous exponents could be interpreted by the model. The exponents are found to be related with the extent of proton transfer when the stretching force constants of the reactive bonds in reactants and products are the same, n^† is constant and λ is high. Isotope effects depend on ΔG°, but their maximum values seem to be related to n^†, being higher when n^†= 0.5 and close to 1 when n^†= 1.0. Proton-transfer reactions in acid–base catalysis are also found to conform with the general picture provided by the model. Suggestions for the molecular parameters which dominated solvent effects are discussed.