Through coupled thermal and orbital calculations including a full description of tidal dissipation, heat transfer and the H 2O sbnd NH 3 phase diagram, we propose a model for the internal structure and composition of Titan testable with Cassini-Huygens measurements. The high value of Titan's orbital eccentricity provides a strong constraint on the amount of the tidal energy dissipation on its surface and within its interior since its formation. We show that only models with a few percent of ammonia (and not zero) in the primordial liquid water shell can limit the damping of the eccentricity over the age of the Solar System. The present models predict that a liquid ammonia-rich water layer should still be present within Titan under an ice I layer, a few tens of kilometers thick. Furthermore, we predict that any event linked to convective processes in the ice Ih layer (like the degassing of methane) could have occurred very late in Titan's history.