Based on X-ray diffraction and electron microscopy it is shown that oxides of the general formula La Ba 2Cu 3O 7-δ become tetragonal when δ deviates slightly from 0. This tetragonal structure is similar to that of La 3- xBa 3+ xCu 6O 14+δ, with a cubic perovskite subcell and triple periodicity. Electron micrographs of these tetragonal oxides show 90° microdomains. Orthorhombic LaBa 2Cu 3O 7-δ with high Tc (∼77 K) is found only when δ ⋍ 0; this sample is subject to formation of twins. Fluorine substitution seems to favor superconductivity.