Abstract The solid electrolyte transition in NaI–AgI was studied by electrical conductivity measurements in the temperature range between 323 K and 523 K. Although NaI does not dissolve in AgI, the addition of NaI led to a stable α–β‐phase boundary of AgI at 420 K, decreased the total ionic conductivity in the α‐phase and caused a smaller step change of conductivity between the values above (α‐phase) and below (β‐phase) 420 K. 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; therefore, we propose a modified version of the Welch–Dienes trial model free energy density in which the quadratic terms for energy are conserved. In this model three terms were considered: the first one associated with the activation energy ( U i ) to promote an atom toward an interstitial site; the second one associated with the attractive interaction due to the Frenkel pair ( U ) and the last one due to the configurational entropy. It was found that the observed step change in conductivity is mainly due to the carrier density n i The trial free energy function admits at least three solutions for given parameter values, but the equilibrium configuration is that in which n i = $ \bar n $ i gives the minimum value at a given temperature. In this way, we were able to reproduce the conductivity changes observed in the NaI–AgI system at 420 K. The phase transition is expected to be first order because the order parameter ( n ) changes abruptly. The activation energy U i associated with this anomaly was of the order of 0.3 eV, and the best fitting parameters were Γ = 2.81, x = 0.77 and γ = 5.49. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)