Enthalpy changes for the reaction of HCl(aq) with Na2WO4(aq) were measured at T = 298.15 K in a HT-1000 calorimeter. The standard enthalpy of reaction for the formation of W7O246-(aq) was calculated on the basis of the experimental results, Delta H-r(m)o(298.15 K) = -(320.7 +/- 1.0) kJ.mol(-1). Combining this with the values from the literature led to the standard enthalpy of formation of W7O246-(aq), Delta H-f(m)o(298.15 K) = -6689.8 kJ.mol(-1). (C) 2001 Academic Press.