A comparison has been made between the experimentally measured value of enthalpy change of the reactionCaO(s, 298 K) + WO3(s, 298 K) = CaWO4(s, 1473 K),and the value for the same reaction derived from the measured enthalpy change of the reactionCaCO3(s, 298 K) + WO3(s, 298.K) = CaWO4(s, 1473 K) + CO2(g, 1473 K).The two values of Delta H, namely 13.97 +/- 2.45 and 9.34 +/- 3.55 kJ mol(-1), are roughly comparable when experimental uncertainties are taken into consideration. This suggests that the error induced in the value of the enthalpy of formation, from the component oxides, of CaWO4 by using CaCO3 instead of CaO as one of the reactants in direct synthesis experiments would not be very great. The lower value of Delta H, 9.34 +/- 3.55 kJ mol(-1), indicates that the CO2 formed in the reaction between CaCO3 and WO3 is released at a temperature somewhat lower than the calorimeter temperature.For all partial pressures of CO2, SrCO3 and BaCO3 decompose at much higher temperatures than CaCO3. Hence, it can be inferred that the agreement between the two values of Delta H would be significantly better for SrO-SrCO3 and BaO-BaCO3, than for CaO-CaCO3.In a recent communication (Q. Guo and O.J. Kleppa, Thermochim. Acta, 288 (1996) 53-61), we assumed that the CO2 formed from SrCO3 + WO3 and BaCO3 + WO3 in the high-temperature calorimeter is released at or very near the calorimeter temperature. Under this assumption, which we believe is valid, the enthalpies of formation for SrWO4 and BaWO4, from the component oxides, reported by these authors should be reliable.