A quasi-elastic neutron scattering (QENS) study of the Ag+-ion diffusion coefficient, D, in glassy (Ag2S)0.5 (GeS2)0.5 has been performed. The results indicate that the magnitude of D is larger, and its activation energy is smaller, than the corresponding quantities inferred from measurements of the d.c. ionic conductivity. This behaviour is ascribed to the fact that QENS probes diffusive processes occurring at very high frequencies (∼1011Hz) where the magnitude of the a.c. conductivity is correspondingly larger, and its temperature dependence is less, than that of the d.c. conductivity.