Composites (1-x)AgI-xAl(2)O(3) were obtained using mechanical treatment of pure AgI with highly dispersed alumina (specific area 270 m(2)/g) in planetary ball mill followed by sintering at 570-870 K. As the concentration of alumina rises, properties of the composites change : both temperature of the superionic phase transition and the heat of the transformation decreases. Average grain size of AgI diminishes with x and at x > 0.6 becomes less than 100 nm, hence, the composites may be attributed to nanocomposites. The phase transition, being very sharp in AgI, becomes diffusive for x > 0.5, the absolute values of the conductivity of both phases approaches each other. All the facts mentioned may be explained by strong interface interaction between Al2O3 and AgI in the nanocomposites leading to a stabilization of a metastable high-disordered state of AgI at low temperatures. Properties of the composites are shown to be sensitive to agents adsorbed on the alumina surface. Silver iodide occurring in a metastable state within the composite may be easily transformed to stable beta-AgI by treatment with water vapour. It is likely that water molecules substitute for Ag+ ions weakly bonded to surface oxygen atoms of alumina, destroying the AgI-Al2O3 interfaces.