The mechanism of ionic transport in PAN based polymer electrolytes with LiBF4 as the Li salt and Al2O3 as ceramic filler is investigated employing a range of advanced solid state NMR methodologies with a special focus on the role of the DMSO employed as a solvent. The residual amount of DMSO present in the samples was found to be virtually independent of the PAN:LiBF4 ratio, but solely dependent on the amount of Li salt (1-2 DMSO molecules per equivalent of Li salt). Only a marginal fraction of Li cations seems to be associated with the PAN polymer, identified from a considerable strong Li-7-H-1 dipolar coupling as exemplified by Li-7-(H-1)-CPMAS-REDOR NMR spectroscopy. The majority of Li cations however is involved in a Li(DMSO)(1-2) complex and exhibits a rather high mobility which persists down to temperatures of T = 220 K. the same temperature, at which the DMSO and BF4- mobility freezes. (C) 2011 Elsevier B.V. All rights reserved.