The electrical conductivity spectra of three mixed alkali glass systems 0.3[xLi(2)O center dot(1 - x)R2O] center dot 0.7B(2)O(3) (with R = Na, K, Rb; x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) have been studied. For the first time it is reported that the strength of the mixed alkali effect does not always increase with the size difference of the involved alkali ions. The experimental results have been successfully interpreted by the structural model proposed by Swenson and his coworkers in combination with the structural considerations. (c) 2005 Elsevier B.V. All rights reserved.