Tetrahedral chain ordering in the Sr 2 MnGaO 5 structure is studied using electron diffraction (ED) and high-resolution electron microscopy. The ED patterns show the presence of satellite reflections, which indicate a commensurately modulated structure with a=5.4056(8)A, b=16.171(3)A, c=5.5592(7)A, q=1/2c * , superspace group Imma(00γ)s00. The superstructure arises due to ordering of the two types of symmetry related tetrahedral chains (L and R) according to a ...LRLR... sequence, where L and R chains alternate along the c-axis within the same (GaO) layer. Numerous defects at different structural levels were observed, comprising interleaving L and R chains, violation of the ...LRLR... chain sequence within one layer, different stacking modes of the ...LRLR... ordered layers with subsequent alternation of blocks of different width along the b-axis of the brownmillerite subcell and island fragmentation of the modulated superstructure. By in situ heating ED experiments it is found that the long-range ordering of the tetrahedral chains is stable up to 665 o C and is completely suppressed at 905 o C.