Substitution of divalent cadmium for monovalent silver in Ag3SBr can be effected at low concentrations. Room temperature X-ray diffraction measurements confirm that uniform solid solutions of Ag3-2xCdxSBr are formed within the composition range 0 less-than-or-equal-to x less-than-or-equal-to 0.10. The effect of Cd substitution in superionic Ag3SBr is minimal. However, in the covalent phase of this compound, Cd substitution significantly reduces the activation energy for ionic mobility as a result of the cationic vacancies which accompany chemical replacement.