Abstract The molar enthalpies of solution of HfTe 3 O 8 , TeO 2 and HfF 4 in 10 mol dm −3 HF(aq) have been measured using an isoperibol-type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of HfTe 3 O 8 (s) has been calculated to be Δ f H m 0 (298.15 K) = −2129.1 ± 10.1 kJ mol −1 . This value of enthalpy of formation of HfTe 3 O 8 is consistent with the free energy of formation of HfTe 3 O 8 determined in this laboratory by the transpiration technique.