Inorganic ceramics- and organic polymer-based solid electrolytes (SE) could revolutionize battery technology because of their nontoxicity, stability during operation and enhanced safety. Of particular interest are electrolytes, which contain high concentrations of ion charge carriers with a minimum polymer concentration, and enable the major ion-conduction path through the inorganic material. In this article we compare ion-conduction mechanisms in two solid electrolytes composed of either an inert or active ceramic matrix with imbedded LiI:P(EO)n electrolytes. On the basis of AC-impedance and NMR data, we suggest that the high-ionic-conductivity (0.5 mS/cm) and low-activation-energy (2.3 kJ/mol) ion paths are brought about by the grain boundaries between the excess of LiI and inert LiAlO2 ceramic nanoparticles. Both confined-in-ceramic polymer electrolyte (PE) and ceramic LiAlO2 grains impede the total ion mobility. The fast ion transport in polymer-in-ceramic electrolytes composed of high-conductivity active Li10SnP2S12, goes through lithium-iodide-rich glass ceramics, and is restricted by slow ion transport via the imbedded polymer electrolyte. Unexpectedly, it was found that at 1:3 salt-to-polymer ratio, the contribution of grain-boundary conductivity in an inert-ceramic–based composite electrolyte is stronger than that of bulk conductivity via active ceramic matrix. One of the possible reasons of the reduced relative contribution of the active ceramics to the total conductivity of polymer-in-ceramic electrolyte is that the ceramic powder was not densified.