New compounds of phthalic acid, Cs(HPHT), and terephthalic acid, Cs-2(TPA), have been synthesized. The enthalpy of solution of Cs(HPHT) in water was determined and combined with the standard molar enthalpies of formation of CsOH(aq), H2O(1) and phthalic acid(s) to calculate the standard molar enthalpy of formation of Cs(HPHT) of -(1035.6 +/-0.5)kJmol(-1). The enthalpies of solution Of Cs-2(TPA) and TPA in approximately 0.11 mol dm(-3) CsOH were determined and combined with the standard molar enthalpies of formation of TPA(s), H2O(1) and CsOH(aq, 1:500) to calculate the standard molar enthalpy of formation Of Cs-2(TPA) of -(1266.2 +/- 0.3) kJ mol(-1). (c) 2005 Elsevier B.V. All rights reserved.