Bismuth tungstates have attracted lot of attention due to their excellent intrinsic properties, such as ferroelectric-piezoelectricity, catalytic behavior, nonlinear dielectric susceptibility and photo catalytic activity. Thermodynamics plays an important role in predicting long term stability of these materials under different reactive conditions. The present paper describes determination of standard molar enthalpies of formation of Bi2WO6(s) and Bi2W2O9(s) compounds employing a high temperature solution calorimeter. The reaction enthalpies of Bi2O3(s), WO3(s), Bi2WO6(s) and Bi2W2O9(s) in liquid Na2O + MoO3 solvent (3: 4 M ratios) at 983 K were measured using a Calvet Calorimeter. The standard molar enthalpy of formation of the compounds at 298 K (Delta H-f(298)degrees) were calculated by combining the experimentally determined values of the reaction enthalpies with the auxiliary data from the literature. The values of Delta H-f(298)degrees of Bi2WO6(s) and Bi2W2O9(s) at 298 K were found to be -(1426.0 +/- 7.9) kJ . mol (1) and -(2379.7 +/- 6.8) kJ . mol(-1), respectively. (C) 2013 Elsevier Ltd. All rights reserved.