The molar enthalpies of solution of ThMo2O8, Mo, and ThF4 in (10 mol dm(-3) HF(aq) + 4.41 mol dm(-3) H2O2(aq)) have been measured using an isoperibol calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of ThMo2O8 has been calculated to be Delta(f)H(m)(0)(298.15 K) = (-2742.2 +/- 4.5) kJ mol(-1). This value of enthalpy of formation of ThMo2O8 is consistent with the second law enthalpy of formation of TnMo(2)O(8) determined in this laboratory by transpiration technique.