Calcium and magnesium oxide are important components of metallurgical slag systems. However, the literature values for the standard enthalpy of formation ΔfH298∘ of both oxides exhibit large variations in some cases. Since ΔfH298∘ is crucial for the modeling and prediction of equilibrium states and, thus, also for process optimization; it was determined by Knudsen effusion mass spectrometry (KEMS). Pure CaO as well as MgO were investigated in an iridium Knudsen cell. For this purpose, the intensities of the main species present in the gas phase were recorded in a temperature range between 1825 K to 2125 K and 1675 K to 2075 K, respectively, and their partial pressures were obtained. It was observed that CaO and MgO evaporated congruently with the main species in the gas phase, Ca, Mg, O, and O2. The experimental vapor pressures of the gas species in the study of MgO are in good agreement with the calculated values using FactSageTM 7.3 and the FactPS database, while those for the evaporation of CaO show significant differences. These calculations are based on available thermodynamic information, including the Gibbs energy functions of CaO(s), Ca(g), MgO(s), Mg(g), O(g), and O2(g). After calculating the partial pressures and equilibrium constants of reactions, an average formation enthalpy of ΔfH298∘ = − 624.5 ± 3.5 kJ/mol for CaO(s) and ΔfH298∘ = − 598 ± 10 kJ/mol for MgO(s) based on the third law method of thermodynamics were obtained. The deviation of ΔfH298∘ for MgO from the previous literature values can be attributed to the use of different ionization cross sections, temperature calibration, and variation of tabulated Gibbs energy functions.