An on-line solution-reaction isoperibol calorimeter has been constructed. The performance of the apparatus was evaluated by measuring the molar enthalpy of solution of KCl in water at 298.15 K. The uncertainty and the inaccurary of the experimental results were within +/-0.3% compared with the recommended reference data. Using the calorimeter, the molar enthalpies of reaction for the following two reactions: LaCl(3)(.)7H(2)O(s)+2Hhq(s)+NaAc(s) = La(hq)(2)Ac(s)+NaCl(s)+2HCl(g)+7H(2)O(l) and PrCl(3)(.)6H(2)O(s)+2Hhq(s)+NaAc(s) = Pr(hq)(2)Ac(s)+NaCl(s)+2HCl(g)+6H(2)O(l), were determined at T = 298.15 K, as -(78.3 +/- 0.6) and -(97.3 +/- 0.5)kJ mol(-1), respectively. From the above molar enthalpies of reaction and other auxiliary thermodynamic quantities, the standard molar enthalpies of formation of La(hq)2Ac and Pr(hq)2Ac, at T = 298.15 K, have been derived to be -(1535.5 +/- 0.7) and -(1536.7 +/- 0.6) kJ mol(-1), respectively.