Molecular dynamics simulations were carried out at 700 K in single alkali and mixed alkali metasilicate glasses to reproduce the dramatic mixed alkali effect occurring in the dilute foreign alkali region. Both small numbers of frozen ions and another kind of alkali appears to immobilize a large number of host ions as previously found in experiment. This large number can now be rationalized as being due to blockage by the less mobile foreign ions of the cooperative ion dynamics that originate from ion‐ion interaction and correlation. © 2004 American Institute of Physics