The first-order phase transition that leads to the superionic phase in AgI-based materials is studied by dc-conductivity measurements and a free energy model. By properly adjusting the model parameters, an abrupt change of disordering concentration, Delta(eta) over bar, is predicted at a transition temperature, T-t. The temperature dependence of the dc-conductivity, sigma(T). is well fitted to the (eta) over bar (T) equilibrium configuration obtained from the trial free energy function. The reported comparative study was done using an AgI-KI modified sample. The model also predicts a transition temperature, T-c for a continuous phase transition (Delta(eta) over bar = 0). (C) 2009 Elsevier B.V. All rights reserved.