The ionic conductivity of montmorillonite (MMT) and MMT/alkali salt mixtures were investigated in the temperature region of RT-400°C by the ac impedance technique. Sodium- or lithium-ion-type MMT showed low conductivity (10-7 to 10-6 S cm-1 at 200°C). The alkali salts (NaSCN, LiI, etc.)/MMT systems showed an enhancement in ionic conductivity similar to the LiI/γ-Al2O3 system. Among the systems investigated, the NaSCN/MMT system showed the highest conductivity (10-4 S cm-1 at 200°C) at the composition where NaSCN occupied completely the Na+ ion sites available in MMT. This characteristic feature is considered to be due to the intercalation ability and/or thin film forming ability of MMT.