In the cooling process of multicomponent Coulomb liquids, the localization of constituent ions is accomplished through various processes. In the superionic system, only a part of the constituent ions freezes at Tgand residual ‘‘mobile’’ ions still remains in the liquid state. The localization of the mobile ions occurs at temperatue region far below Tg. The localization of less‐mobile ion is detectable by thermal relaxation technique such as DSC, but no dispersion is seen in dielectric response. On the other hand, the localization of mobile ions can be detected only by dielectric response as a power‐low frequency dependence of conductivity &sgr;[&ohgr;]∼(i&ohgr;&tgr;c)n(0<n<1), and increase in the relaxation time &tgr;c.Self‐consistent semi‐quantitative analysis is given, based on the excess‐free‐volume model, to calculate the temperature and frequency dependence of conductivity and permittivity, as well as specific heat change through Tg. Also, a brief discussion is given on the mode‐coupling approach by Sjo¨gren.