Calorimetric enthalpies of reaction have been measured for the following enzyme-catalysed reactions at the temperature 298.15 K: prephenate(aq) = phenylpyruvate(aq) + carbon dioxide(aq), prephenate(aq) + NAD(ox) (ag) + H2O(1) = 4-hydroxyphenylpyruvate(aq) + NAD(red)(aq) + carbon dioxide(aq). Here, NAD(ox) and NAD(red) are, respectively, the oxidized and reduced forms of beta-nicotinamide adenine dinucleotide. The enzymes that catalyse these respective reactions, prephenate dehydratase and prephenate dehydrogenase, were prepared by expression of the appropriate plasmids using the techniques of molecular biology. The calorimetric measurements together with the equilibrium modeling calculations lead to a standard molar enthalpy change Delta(r)H(m)(o) = -(126 +/- 5) kJ.mol(-1) for the reference reaction: prephenate(2-)(aq) = phenylpyruvate(-)(aq) + HCO3-(aq). Similarly, Delta(r)H(m)(o) = -(74 +/- 3) kJ.mol(-1) for the reference reaction: prephenate(2-) (ag) + NAD(ox)(-)(aq) + H2O(l) = 4-hydroxyphenylpyruvate(-) (ag) + NAD(red)(2-)(aq) + HCO3-(aq) + H+(aq). Both results pertain to T = 298.15 K and ionic strength I = 0. Benson estimates for the entropies lead to approximate values of the equilibrium constants K approximate to 1 . 10(26) and K approximate to 1 . 10(12), respectively, for the above two reference reactions.