Hydrogen absorption/desorption of Ti3Al-based alloys with α2 single-phase, β0 single-phase and β/O two-phase microstructures was investigated to obtain systematic understanding microstructure effects. On exposing all the alloys to hydrogen, β (βH) and γ (γH) hydrides are formed. The βH and γH have bcc and bct (close to fcc) based structures, respectively. The hydrogen/metal ratio of the γH is larger than that of βH. Thus, solute hydrogen in Ti3Al based alloys stabilizes the bcc-based phase just like Nb, regardless of their original structure before hydrogenation. On the other hand, reversible hydrogen absorption/desorption around 100°C and related reversible βH-γH transformation was observed in β/O two-phase (Ti,Nb)3Al and α2-Ti3Al alloys, but not obviously observed in a β0-(Ti,Nb)3Al alloy. In forming the βH, the expansion of the β0 and β phases occurred isotropically keeping their structures B2 and bcc, respectively. In contrast, the O and α2 phases expanded in anisotropic manner. The lattice deformation for the βH→γH transformation can be described similarly to those for the O→βH and α2→βH transformations. Such lattice deformation in anisotropic manner most likely agrees with that observed in a martensitic displacive transformation, because of the surface relief observed on the γH after the βH→γH transformation in the β0-(Ti,Nb)3Al alloy and many twins formed during the α2→βH transformation in the α2-Ti3Al alloy. Mechanisms of the reversible βH-γH transformation in the β/O two-phase (Ti,Nb)3Al and α2-Ti3Al alloys was proposed on the basis of similarities in the lattice deformation and ordering of hydrogen atoms required for the βH-γH, O-βH and α2-βH transformations.