Ionic conductivity, sigma(i), of dilute pseudobinary alloys (CuBr)(1-x)(Cu2Se)(x) (x less than or equal to 0.1) in their gamma-phase has been measured by an ac method. The increase of the ionic conductivity propertional to x has been observed, which is attributed to interstitial ions brought by Cu2Se dissolved in CuBr. It is found that the temperature dependence of mobility of interstitial ions, mu, evaluated by the relation Delta sigma(i)/x = k mu (k is a constant) is bent at the temperature corresponding to the extrinsic-intrinsic transition of the based material CuBr.