The molar enthalpies of solution of UTeO5, UTe3O9, TeO2, and UO3 in HCl (aq, c = 11 mol . dm(-3)) have been measured by using an isoperibol calorimeter From these results and other auxiliary data, the standard molar enthalpies of formation Delta(f)H(m)(o)(298.15 K) of UTeO5(cr) and UTe3O9(cr) have been calculated to be -(1606.3 +/- 3.5) kJ . mol(-1) and -(2287.2 +/- 9.5) kJ . mol(-1), respectively. These values of enthalpies of formation of UTeO5 and UTe3O9 are consistent with the second-law enthalpies of formation of these compounds derived from their vapour pressures determined in this laboratory by a transpiration technique.