Mass-transport properties of anodic alumina membranes exploited in a number of technological areas are strongly affected by the real pore structure and arrangement of channels that can split or terminate during the anodization process. This paper focuses on the investigation of pore branching and rearrangement caused by voltage variation in the course of the anodic oxidation of aluminum. Gas-transport measurements were utilized for the quantitative determination of an effective through porosity of multilayer anodic alumina membranes with branched channels obtained by variation of anodization voltage. It was shown that on decrease of anodization voltage a branching of pores occurs, while an increase of anodization voltage leads to the termination of some of the pores with an increase in the diameter of others. Gas permeance measurements combined with electron microscopy unambiguously prove dead-end pore formation on voltage increase, while no pore merging appears. This generally affects any mass-transport properties and applications of anodic alumina membranes as the delivery of any species (e.g. ions, gas molecules, etc) through the blocked channels is impossible.