Solid-state batteries are seen as key to the development of safer and higher-energy-density batteries, by limiting flammability and enabling the use of the lithium metal anode, respectively. Composite polymer–ceramic electrolytes are a possible solution for their realization, by benefiting from the combined mechanical properties of the polymer electrolyte and the thermal stability and high conductivity of the ceramic electrolyte. In this study we used different liquid electrolyte chemistries as models for the polymer electrolytes, and evaluated the effect of adding a variety of porous and dense ceramic electrolytes on the conductivity. All the results could be modelled with the effective medium theory, allowing prediction of the conductivity of electrolyte combinations. We unambiguously determined that highly conductive porous particles act as insulators in such systems, whereas dense particles act as conductors, thereby advancing our understanding of composite electrolyte conductivity.