The correlation between the local structure of alkali glasses and their ionic conductivity is investigated by a bond valence analysis of reverse Monte Carlo (RMC) fits to experimental diffraction data. Our previously detected general relation between the volume fraction of the transport pathways and their ionic conductivity also permits to reproduce the mixed alkali effect and provides further insight into its structural origin. The drastic drop of the conductivity is mainly caused by the blocking by immobile unlike cations, which is highly effective due to the low dimensionality of the pathways on local length scales. Factors that affect the pathway dimensionality are discussed. The evolution of the transport pathways in time is studied by applying the BV approach to entire molecular dynamics trajectories. (C) 2004 Elsevier B.V. All rights reserved.