The response of the natrolite crystal structure (Na16Al16Si24O80 · 16H2O) to dehydration/rehydration was evaluated as a function of PH2O with X-ray powder diffraction (XRD) data measured in situ from 23 to 400 °C. Dehydrated natrolite at a low-PH2O appears as a mixture of two anhydrous phases, the previously described α1-metanatrolite (F112, a = 16.177(1) Å, b = 16.943(1) Å, c = 6.4370(4) Å, γ = 89.685(2)°, V = 1764.3(2) Å3), and a new phase, α2-metanatrolite (Fdd2, a = 17.576(1) Å, b = 18.163(1) Å, c = 6.3704(4) Å, V = 2033.7(2) Å3). The structure of α2-metanatrolite was solved using a combination of powder pattern indexing, distance least-squares modeling, and Rietveld refinement. The structures of α1- and α2-metanatrolite provide an excellent context for describing the behavior of natrolite phase transitions and the effects of PH2O. Both metanatrolite phases occur simultaneously during dehydration under low-PH2O conditions and they re-adsorb H2O molecules at different rates during rehydration. α2-metanatrolite adsorbs H2O molecules and reconverts to natrolite more rapidly than α1-metanatrolite because of the larger channel size in the α2-metanatrolite framework. The path dependence of natrolite phase transitions and the dependence on PH2O illustrate the sensitivity of the natrolite framework and its extraframework Na cations to hydration state, with α1- and α2-metanatrolite forming under low-PH2O conditions and only α1-metanatrolite forming under high-PH2O conditions. The flexibility of the natrolite framework makes the natrolite- to- α1-/α2-metanatrolite-to-natrolite reaction reversible, although strain-induced mosaicity resulting from the phase transition is not reversible.