In our preceding paper (Y. Fukui et al., Phys. Chem. Chem. Phys., 2023, 25, 25594–25602), we reported a systematic study of the Ag⁺-ion conducting behaviour of silver iodide (AgI)-loaded mesoporous aluminas (MPAs) with different pore diameters and AgI-loading ratios. By optimising the control parameters, the Ag⁺-ion conductivity has reached 7.2 × 10⁻⁴ S cm⁻¹ at room temperature, which is more than three orders of magnitude higher than that of bulk AgI. In the present study, the effect of silver bromide (AgBr)-doping in the AgI/MPA composites on Ag⁺-ion conductivity is systematically investigated for the first time, using variable-temperature powder X-ray diffraction, differential scanning calorimetry, and electrochemical impedance spectroscopy measurements. The AgBr-doped AgI/MPA composites, AgI-AgBr/MPA, formed a homogeneous β/γ-AgI-structured solid solution (β/γ-AgI_ss) for the composites with AgBr ≤ 10 mol%, above which the composites underwent a phase separation into β/γ-AgI_ss and face-centred cubic AgBr solid solutions (AgBr_ss). The onset temperature of the exothermic peaks attributed to the transition from α-AgI-structured solid-solution phase to β/γ-AgI_ss or AgBr_ss decreased with increasing the AgBr-doping ratio. The room-temperature ionic conductivity of the AgI–AgBr/MPA composites exhibited a volcano-type dependence on the AgBr-doping ratio with the highest value (1.6 × 10⁻³ S cm⁻¹) when the AgBr content was 10 mol%. This value is more than twice as high as that of the highest conducting AgI/MPA found in our previous study.