The tobermorite supergroup is composed by a number of calcium-silicate-hydrate (C-S-H) minerals characterized by different hydration states and sub-cell symmetries. Taking into account their basal spacing, closely related to the hydration state, phases having a 14 Å (plombièrite), 11 Å (tobermorite, kenotobermorite, and clinotobermorite), and 9 Å (riversideite) basal spacing have been described. Tobermorite and kenotobermorite belong to the so-called tobermorite group and differ for their thermal behaviour which can be “normal” (the phase shrinks to a 9 Å phase at 300 °C) or “anomalous” (the phase preserves its 11 Å basal spacing at 300 °C). Specimens of Al-rich tobermorite from Montalto di Castro and Vallerano, Latium, Central Italy, showing a “normal” thermal behaviour, were studied in order to describe the transition from the 11 Å to the 9 Å phase by means of thermo-gravimetric–differential scanning calorimetry (TG-DSC) analyses as well as in situ and ex situ X-ray diffraction experiments. The TG-DSC analyses showed a continuous mass loss from 100 °C up to 700 °C, with different mass loss gradients between 100 °C up to 300 °C and between 300 °C up to 700 °C, corresponding to the dehydration of tobermorite and dehydroxylation of “tobermorite 9 Å”, respectively. Above 700 °C, “tobermorite 9 Å” is replaced by wollastonite. The X-ray powder diffraction data were collected at the GILDA beamline of the ESRF, Grenoble, France, from room temperature up to ca. 840 °C. Tobermorite is completely replaced by the 9Å phase at ca. 300 °C, whereas the latter is transformed into wollastonite at ca. 700 °C. The transition from the 11 Å to the 9 Å phase seems to be favoured by the transient appearance of a clinotobermorite-like compound.