Using the data obtained by Knudsen effusion mass spectrometry, the standard formation thermodynamic properties of La₂Hf₂O₇, Nd₂Hf₂O₇, and Gd₂Hf₂O₇ were calculated in the present study at high temperatures. Based on the results obtained, it was shown that the standard formation Gibbs energies of La₂Hf₂O₇, Nd₂Hf₂O₇, and Gd₂Hf₂O₇ from the elements at the temperature 2445 K were consistent with the empirical rule concerning decrease of stability of pyrochlore hafnate phase with decrease in lanthanoid ionic radius. The La₂Hf₂O₇ and Gd₂Hf₂O₇ heat capacities were obtained in the present study by differential scanning calorimetry. These data were used along with those found earlier to evaluate the standard formation Gibbs energies of La₂Hf₂O₇ and Gd₂Hf₂O₇ from the elements at the temperature 298 K, which equal (-3937 ± 10) kJ/mol and (-3895 ± 10) kJ/mol, respectively. The thermodynamic properties of La₂Hf₂O₇, Nd₂Hf₂O₇, and Gd₂Hf₂O₇ estimated in a wide temperature range allowed consideration of reliability of data available in the literature.