Rapid voltage reduction across porous anodic coatings results in zero current, and a relatively long time is required for recovery to the new equilibrium current. The recovery time is dependent on the voltage change, rate of change, and electrolyte temperature. Capacity measurements show that physical thinning occurs only when current begins to increase late in the recovery period. This behavior appears inconsistent with the conventional pore base dissolution mechanism of porous oxide growth. © 1968, The Electrochemical Society, Inc. All rights reserved.