Nominally pure polycrystalline samples of Ag3-xCuxSBr in the composition range x = 0 --> 0.25 were prepared from the melt and annealed at 473 K for 6 months. For the x = 0 compound, faint traces of unreacted AgBr can be detected by X-ray diffraction. As composition parameter x increases in the Cu-substituted samples, the cubic structure of Ag3SBr is gradually lost while the proportion of unreacted silver halide grows. We investigate the effect of Cu substitution on the ionic conductivity of Ag3SBr. With only small quantities of Cn replacing Ag in this compound, one observes a large reduction in the magnitude of the first-order conductivity change associated with the beta-gamma phase transition at 126 K. Furthermore, the activation energy governing Ag migration is increased in both the high-temperature beta phase and the low-temperature gamma phase with increasing Cu substitution.