When ac measurements were made on a glass using ion-blocking electrodes, a positive ion-depleted region developed near the temporary anode during each halfcycle when the temperature was high or when the frequency was low. As a result, even the conductance at the plateau (from which the de conductivity was evaluated) was no longer constant, but gradually decreased with decreasing frequency. At lower frequencies than that of the plateau, the conductance drastically decreased and the susceptance gave the peak maximum. For the purpose of characterizing this region, the depletion index, eta, was introduced and its variations with glass composition and temperature explained.