The influence of the microstructural changes during the crystallization in the glassy silver ion conducting system AgI-Ag2O-V2O5 has been studied by impedance spectroscopy and high temperature XRD. A non-destructive procedure has been used for the in situ determination of the glass crystal interface area within the forming composites. The variation of the interface area is nearly proportional to the observed anomalous increase in the conductivity during crystallization, suggesting that an interfacial mechanism is responsible for the conductivity enhancement.