Activation enthalpies of ionic conductivities in Na-Rb alumino-germanate and borate glasses are reviewed. Correlations between the activation enthalpy and the ratio of average distances between like alkali ions, < d(ion)>, to the average distances between network-former atoms, < d(network)>, are elucidated for single- and mixed-alkali glasses. The Haven ratio is shown to decrease with decreasing < d(ion)>/< d(network)>. Interstitial-like and substitutional-like subnetworks of ion sites are suggested. The experimentally observed dependence of the Haven ratio on < d(ion)>/< d(network)> is consistently reproduced by a Monte Carlo simulation of ion dynamics on this random network including single and collective ion jumps. (c) 2005 Elsevier B.V. All rights reserved.