We report on abnormal behavior in anodic oxidation of Al in mechanically confined structures for formation of horizontal nanoporous anodic alumina oxide (H-AAO). Instead of smooth pore walls, periodic dendrite inner pore structures form, the growth rate is suppressed to 5% of its value during bulk anodization under the same conditions, and a steady-state is never reached. These anomalies associated with H-AAO originate from suppressed volume expansion and a plastic flow of Al2O3 confined by the SiO2 hard mask. By determining new anodization conditions leading to zero volume expansion, dendritic H-AAO can be avoided and kinetic retardation minimized. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3532778] All rights reserved.