The solid electrolyte transition in 0.1 NaI-0.9AgI was studied by electrical conductivity measurements. Although NaI does not dissolve in AgI, the addition of NaI led to stable beta-to-alpha phase boundary of AgI at 420 K, decreased total ionic conductivity of alpha-AgI and decreased difference between values of conductivity in beta- and alpha-AgI. However, the magnitude of the step change in conductivity (Deltasigma) at 420 K showed variation with high temperature anneals of the sample. The conductivity data were analyzed by considering that the ionic carrier density is the main source of the sudden increase in the ionic conductivity through this solid electrolyte transition. Thus, by using a previously proposed trial free-energy density for the equilibrium defects concentration, we were able to reproduce the conductivity change observed in 0.1NaI-0.9AgI at 420 K. (C) 2004 Elsevier B.V. All rights reserved.