[en] A careful TEM and XRD study of the (Ba_1-xLa_x)₂In₂O_5+x, 0≤x≤0.6, 'defect-perovskite'-type solid solution has been carried out. A well-defined structural phase transition is shown to occur between x=0.1 and 0.2 from the orthorhombic brownmillerite structure type on the low x side to a multiple twinned, tetragonal 1x1x2 perovskite-related superstructure phase on the high x side at x=0.2. This phase transition correlates with an important phase transition previously observed in electrical conductivity versus temperature measurements. The existence of additional satellite reflections close to the G±((1)/(2)) <001>_p^* regions of reciprocal space was found to be typical of all (Ba_1-xLa_x)₂In₂O_5+x specimens, although their intensity relative to the parent Bragg reflections systematically reduces as x increases. As x increases beyond 0.2, the G±((1)/(2)) <001>_p^*-type satellite reflections initially become weaker and rather more diffuse for x=0.3 before splitting into pairs of rather weak and somewhat diffuse incommensurate satellite reflections for x=0.4 and beyond. An interpretation in terms of oxygen vacancy ordering and associated structural relaxation is given. Additional structured diffuse scattering is also observed and a tentative explanation in terms of Ba/La ordering and associated local strain distortions put forward