Correlation effects in the ionic conductivity of gamma-Ag0.7Cu0.3I were studied by impedance spectroscopy measurements in the frequency range 5 Hz-13 MHz and below the superionic transition of AgI at 147 degrees C. As the temperature decreases from 147 degrees C, the spectra of the real part of the conductivity (sigma') shows a transition from their dc values to a dispersive regime where the conductivity shows a power-law frequency dependence at higher frequencies, as a consequence of correlation effects among charge carriers. The dielectric permittivity (epsilon*) shows frequency dependence that may be related to dielectric relaxation of the material as a consequence of ionic hopping. Also, the modulus spectra shows asymmetric peaks that have been analyzed in terms of non-exponential decay function, with a Kohlrausch parameter beta that increases from 0.8 at 22 degrees C to 1.0 at 122 degrees C In contrast to single cation AgI-systems which follow a temperature invariant shape, the observed behavior can be explained by a mixed-cation effect due to enhancement of Cu+ mobility with increasing temperature. Both the dc conductivity (sigma(0)) and the crossover frequency (w(p)) to the power-law dependent regime of sigma' (w) were found to be thermally activated with the same energy, E-sigma, pointing to a common origin of them in ionic hopping. (c) 2004 Elsevier B.V. All rights reserved.