We study ionic transport in nano- and microcrystalline (1−x)Li2O:xB2O3 composites using standard impedance spectroscopy. In the nanocrystalline samples (average grain size of about 20 nm), the ionic conductivity σdc increases with increasing content x of B2O3 up to a maximum at x≈0.5. Above x≈0.92, σdc vanishes. By contrast, in the microcrystalline samples (grain size about 10μm), σdc decreases monotonically with x and vanishes above x≈0.55. We can explain this strikingly different behavior by a percolation model that assumes an enhanced conductivity at the interfaces between insulating and conducting phases in both materials and explicitly takes into account the different grain sizes. © 2000 The American Physical Society.