Inconsistencies in experimental themiochemical analysis of the anatase to rutile phase transition have led to various studies in order to elucidate the physical and chemical parameters affecting the stability of TiO2 at the nanoscale. Using a thermodynamic model, we present predictions of the transition enthalpy of nanocrystalline anatase and rutile as a function of shape, size, and degree of surface passivation, showing that thermochemical results can differ for various faceted or spherical nanoparticles.