The microstructure of the n = 3 member of the homologous series of superconductors GaSr 2 Ca n - 1 Cu n O 2 n + 3 prepared under high pressure (6 GPa) was investigated by means of selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The GaSr 2 Ca 2 Cu 3 O 9 + δ phase has an orthorhombic structure with a double periodicity along the b-axis, due to the intralayer ordering between two different types of GaO 4 chains. Also, interlayer disordering of the chains along the c-axis was observed.