In this study, the base 0.1Ga(2)S(3)+0.9GeS(2) was chosen because it was found that it could be combined with all of the alkali sulfides to yield glasses over the same relatively wide glass-forming composition range. Both M2S and MI, where M=Li, Na, K and Cs, were added to form broad glass-forming composition ranges to systematically study the ionic conductivity in these ionic conducting chalcogenide glasses as a function of the alkali ion radius. As expected, additions of both alkali sulfide MS) and alkali iodide (MI) increase the conductivity of the glass. For the 0.6M(2)S+0.4 (0.1Ga(2)S(3)+0.9GeS(2)) glasses, however, the conductivity only increases slightly with the addition of Nal. The ionic conductivities show significant differences between the Li2S, Na2S and K2S, Cs2S glasses due to the large differences in their activation energies. The alkali radius effect on the activation energy was analyzed and discussed based on the modified Anderson and Stuart model. (C) 2007 Elsevier B.V. All rights reserved.