We present the three coupled rate equations of the mismatch generated relaxation for the accommodation and transport of ions (MIGRATION) concept that describe the ion dynamics in, for example, ion-conducting glasses and simultaneously yield scaled conductivity and permittivity spectra. Here, model spectra are compared with the experimental spectra of xNa(2)O . (1-x)GeO2 glasses for x=0.0513, 0.0902, 0.213. This allows us to estimate the long-time limit of the localized rms displacement, lim (t-->proportional to) { - 6Dt}(0.5), which may be roughly interpreted as the average size of the ion cage before the dynamics become diffusive. For the three compositions considered, we obtain an approximate x(-1/3) dependence of this quantity, which implies that the ion dynamics are largely governed by the Coulomb interactions between the mobile ions. (C) 2004 Elsevier B.V. All rights reserved.