In a keto–enol tautomerisation two protons are transferred. The reaction can be catalysed by acids, by bases and can also take place through a concerted transition state involving both an acid and a base catalyst. A model based on the Marcus relation describes the energetics, the isotope effects, the Brønsted slopes and the pK range over which the concerted mechanism is observed. The concerted route will be more likely for enzyme-catalysed reactions since both the acid and base catalyst may be located near the reaction site. The model is applied to the reaction of dihydroxy-acetone phosphate with triose phosphate isomerase. In this case the transition state is probably a simple base-catalysed transition state, but the whole reaction path may be concerted.