Chiavennite (CaMnBeSi5O13(OH)2 · 2H2O, Z = 4) is a rare Be-bearing zeolite mineral with an interrupted framework of four-connected [SiO4] and three-connected [BeO4] tetrahedra. A sample from the type locality in the Rhetic Alps of Italy has been re-investigated at room temperature in order to clarify the space-group ambiguity discussed in the literature and to explore the system of hydrogen bonds. In addition, the thermal stability of this zeolite was tracked by in situ single-crystal X-ray diffraction between 25°C and 425°C. In spite of the pseudo-orthorhombic cell dimension, the new data indicate that at room temperature chiavennite is truly monoclinic P21/c (β ≈ 90°) and twinned as recently shown for the new mineral ferrochiavennite. The dehydration experiments under dry conditions at different temperatures showed that chiavennite continuously released water up to at least 425°C without topological modification. The loss of more than one H2O molecule at 250°C decreases the Ca coordination from eight- to seven-fold. After release of the first H2O molecule, and strong dynamic disorder of the remaining H2O, chiavennite evolves into orthorhombic symmetry of space group Pbcn. The continuous monoclinic–orthorhombic transition from space group P21/c to Pbcn upon dehydration is attributed to loosening of the system of hydrogen bonds with extraframework H2O as donor and oxygen at the cavity walls as acceptor.