Transmission electron microscopy is used to characterize the defects in highTcsuperconductor, Ba2YCu3O6.9produced by two different ceramic processing procedures. In both samples the microstructure is dominated by lamellar domains. These have been identified as reflection twins by observations both of rotations of the diffraction patterns and of &dgr;‐fringe contrast. The reflection plane is (110). The characteristic width associated with the twins correlates with annealing time. Occasionally within the twinned domain, substructure on a 50‐A˚ scale is visible, which is attributed to further subdivision into twinned and matrix domains. The twin structure does not interrupt the continuity of the Cu‐O planes between Y and Ba layers, but does terminate Cu‐O chains in the inter‐Ba Cu‐O planes. Convergent beam electron diffraction results are consistent with aPmmmspace group. Microcracks and disloction loops are also observed in the (001) basal plane, indicating that the (001) plane is both the slip and cleavage plane in this material.