Magnesium borohydride [Mg(BH4)2] attracts a particular interest as a material for hydrogen storage because of its high gravimetric capacities and being suggested as a rehydrogenable compound. Although extensively studied, besides the whole decomposition process, a large debate is still present in the literature about the temperatures leading to the different (and in many cases, unknown) products. In this paper, an ad hoc designed thermogravimetric study, together with a critical review of literature data, allowed us to identify the products for low reaction rates. Two reaction environments have been considered: dynamic vacuum and hydrogen atmosphere. In order to guarantee quasi-equilibrium conditions, the samples were obtained after 20 h isotherms in the room temperature to 400°C range. The decomposition of γ-Mg(BH4)2 has been here characterized by adopting a new approach and by X-ray diffraction (XRD) and medium-infrared spectroscopy, together with experimental techniques used for the first time for this process (far-infrared and UV-vis-near-infrared spectroscopies). Density functional calculations were performed to help the identification of the amorphous products. A possible process mechanism was delineated and in particular that (a) Mg(BH4)2 decomposition starts at 200°C; (b) MgB4H10 is proposed, for the first time, as the phase responsible for its reversibility for T < 270°C, which would implicitly restrict the Mg(BH4)2 reversible capacity to 3.7 mass %.