Abstract Hydrogenation products of the Zintl phases AeTt (Ae = alkaline earth; Tt = tetrel) exhibit hydride anions on interstitial sites as well as hydrogen covalently bound to Tt which leads to a reversible hydrogenation at mild conditions. In situ thermal analysis, synchrotron and neutron powder diffraction under hydrogen (deuterium for neutrons) pressure was applied to BaTt (Tt=Ge, Sn). BaTtH_y (1<y<1.67, γ-phases) were formed at 5 MPa hydrogen pressure and elevated temperatures (400–450 K). Further heating (500–550 K) leads to a hydrogen release forming the new phases β-BaGeH_0.5 (Pnma, a=1319.5(2) pm, b=421.46(2) pm, c=991.54(7) pm) and α-BaSnH_0.19 (Cmcm, a=522.72(6) pm, b=1293.6(2) pm, c=463.97(6) pm). Upon cooling the hydrogen rich phases are reformed. Thermal decomposition of γ-BaGeH_y under vacuum leads to β-BaGeH_0.5 and α-BaGeH_0.13 [Cmcm, a=503.09(3) pm, b=1221.5(2) pm, c=427.38(4) pm]. At 500 K the reversible reaction α-BaGeH_0.23 (vacuum)⇄β-BaGeH_0.5 (0.2 MPa deuterium pressure) is fast and was observed with 10 s time resolution by in situ neutron diffraction. The phases α-BaTtH_y show a pronounced phase width (at least 0.09<y<0.36). β-BaGeH_0.5 and the γ-phases appear to be line phases. The hydrogen poor (α- and β-) phases show a partial occupation of Ba₄ tetrahedra by hydride anions leading to a partial oxidation of polyanions and shortening of Tt–Tt bonds.