The critical currents and the irreversibility line of bulk melt textured YBa2Cu3O7have been found to evolve with the increase of the oxygenation time. A strong field‐dependent reduction of the critical currents is observed which finally leads to a downward shift of the irreversibility line. Transmission electron microscopy micrographs of aged samples display a huge increase in the concentration of stacking faults near the microcracks and at the YBa2Cu3O7‐Y2BaCuO5interfaces. Low field ac susceptibility measured withH∥abshows a strong reduction of the screening capability thus meaning that nonsuperconducting planar defects parallel to theabplanes have been created. We suggest that an enhanced two‐dimensional behavior of the vortex flux lattice occurs whenH∥cas a consequence of this layered defective structure. ©1995 American Institute of Physics.