Calorimetric enthalpies of reaction have been measured for the following enzyme-catalysed reactions: L-glutamine(aq) + H2O(1) = L-glutamate(aq) + ammonia(aq), L-asparagine(aq) + H2O(1) = L-aspartate(aq) + ammonia(aq). An equilibrium model that accounted for the multiplicity of ionic forms of the reactants and products was used to calculate standard molar enthalpies for reference reactions involving specific species. Values of equilibrium constants for the reference reactions were obtained using results from the literature in thermochemical cycle calculations. The thermodynamic results are discussed in relation to the structural changes involved in these reactions.