High-temperature oxide melt solution calorimetry and Rietveld refinements of powder X-ray diffraction data were used to investigate the structure ( Fd3m; Z=8) and energetics of a series of RE2Ti 2O 7 ( RE=Sm-Lu) compounds with the pyrochlore structure as well as La 2Ti 2O 7 with a layered perovskite-type structure. All of the RE-titanates were found to be stable in enthalpy with respect to their oxides. In the pyrochlore series, Lu 2Ti 2O 7 was least stable in enthalpy (∆ Hf-ox at 298 K=-56.0±4.0 kJ/mol); the most stable materials were Gd-, Eu-, and Sm 2Ti 2O 7 with ∆ Hf-ox at 298 K=-113.4±2.7, -106.1±4.2, -115.4±4.2 kJ/mol, respectively. In general, as the radius ratio of the A- to B-site cations, RA/ RB, decreases, the pyrochlore structure becomes less stable. The trend of ionic radius of the RE3+ cation vs. ∆ Hf-ox at 298 K is non-linear and approximately parallels the increasing "resistance" to ion-beam-induced amorphization as RA/ RB decreases.