Rutile is the thermodynamically stable phase of coarsely crystalline titanium dioxide; however, metastable anatase is frequently present in TiO(2) nanoparticles as a result of the lower surface (solid-vapor) energy of this last polymorph. Here, we show that the presence of a solid-solid interface (frequently present due to aggregation or sintering after synthesis and calcination procedures) also markedly influences the polymorphic stability of TiO(2) at the nanoscale. By revisiting calorimetric data reported on TiO(2) polymorphs, and using a different analysis approach, we derive both surface and interface energies for anhydrous and hydrous interfaces and redraw the stability diagram for anatase and rutile at the nanoscale including an interface term for the first time. The presence of the solid-solid interface (grain boundary) is observed to shift the critical size of the anatase-rutile transition. A similar approach is suggested to be required to provide a better understanding of reported calorimetric data on nanomaterials such as ZrO(2) and Fe(2)O(3).