Chalcohalide glasses in the CdSe–AgI–As2Se3 system were synthesised and characterised for the first time. The glass-forming range for (CdSe)x(AgI)0.5−x/2(As2Se3)0.5−x/2 compositions was found to be limited to x≤0.20 without any significant change in the glass transition temperature but with two sub-regions with decreased (x≤0.05) and increased (x≥0.10) crystallisation ability. The glasses belong to ionic conductors and their conductivity decreases drastically with increasing x accompanied by simultaneous increase of the activation energy. The observed changes in the ionic transport properties are caused by two independent mechanisms: (i) an exchange reaction, CdSe+2AgI⇄CdI2+Ag2Se, taking place in the glass-forming melt, and (ii) blocking of Ag+ preferential conduction pathways by cadmium.