AbstractThe ac electrical conductivity versus temperature dependence of solids undergoing phase transitions incorporating various guest ions, viz isovalent cations and anions, ions with different ionic radius, ions with different polarizability, in Na2SO4-based, Ag2SO4-based, Li2SO4-based and U02SO4-based compositions along with MxTl(1x)I compositions for 1 ≤ x ≤ 0.13, whose M = K, Rb, Cs, are reported. An overview is given of the interplay of various factors, structural and non-structural, with their contribution limits impacting on locking-in the fast cation conductivity phase and the interpretation of fast-ion conductivity in the context of a percolation-type of ionstransport mechanism.