Ionic conductivity of high-temperature phases of alkaline metal perchlorates is studied. It is found that the conductivity passes through a minimum with increasing radius of cation, and KClO4 exhibits the lowest conductivity. This is explained by a decrease in the relative size of conduction channel, which hampers the cation transfer, and an increase in the relative free volume. The free-volume increase promotes the perchlorate anion reorientation and reduces the activation energy for ion transfer by the "paddle-wheel" mechanism, as a result, the conductivity increases. (c) 2007 Elsevier B.V. All rights reserved.