The molar enthalpies of a solution of ZrMo2O8, Mo, and ZrF4 in 10 mol dm(-3) HF(aq)+4.41 mol dm(-3) H2O2(aq) have been measured using an isoperibol-type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of ZrMo2O8 has been calculated to be Delta fH(m)(0) (298.15 K)= -2588.6+/-4.5 kJ mol(-1). This value of enthalpy of formation of ZrMo2O8 is consistent with the second law of enthalpy of formation of ZrMo2O8 determined in this laboratory by the transpiration technique.