Relaxation studies at GHz frequencies using Brillouin scattering demonstrate interactions between the diffusive motions of Ag** plus and high frequency phonons in the fast ion conducting glass (AgI)//0//. //6(Ag//2O plus 2B//2O//3)//0//. //4. For the superionic (AgI)//x(AgPO//3)//1// minus //x glasses, the fast ion diffusion relaxation mode was observed in the liquid state where it is resolved from the structural relaxation for concentrations x greater than 0. 3. The relaxation strength of the fast ion diffusion mode is smaller than that of structural relaxation. The data are consistent with single relaxation time behaviour.