We have applied the concept of heterogeneous doping [1] to prepare and examine composite electrolytes, consisting of silica particles, low molecular weight polyethylene glycol solvents and lithium perchlorate salt. These "soggy sand" electrolytes combine high ionic conductivities (on the order of mS cm(-1)) and high Li transference numbers (typically 60-80%) with improved mechanical properties. They were characterized using differential scanning calorimetry, dc-polarization and ac-impedance spectroscopy, zeta potential measurements and viscosimetry. Oxide, size and concentration as well as solvent molecular weight were varied to better understand the influence of ceramic oxide fillers on the ion conduction in these systems. As regarding the filler content, we observe that both conductivity and transference number of Li+ start increasing already at low volume fractions of oxide particles, reach a maximum and subsequently decrease to low values. The percolating network is - after initial partial coarsening - found to be stable within the time periods of the measurements. (C) 2011 Elsevier Ltd. All rights reserved.