Positron annihilation lifetime spectroscopy (PALS) was performed on a series of polydimethylsiloxane (PDMS)/fumed silicon dioxide (SiO2) composites at temperatures between -185 and 100degreesC to study the effect of filler content and filler particle size on the free volume properties and the positron annihilation characteristics. The glass transition behavior of the PDMS/SiO2 composites was determined with differential scanning calorimetry. A clear influence on the o-Ps lifetime (73) in the polymer upon addition of nano-sized fumed SiO2 was observed at all temperatures. The observed o-Ps lifetime behavior was related to filler content and filler particle size. A transition in the temperature dependence of the o-Ps lifetime was observed close to -35degreesC above which temperature PDMS exhibits long o-Ps lifetimes. A relationship between tau(3) and the surface tension, equivalent to the behavior of ordinary molecular liquids was observed in this temperature region. The o-Ps yield was strongly reduced in the crystallization region and by addition Of SiO2. The reduction due to filler addition did, however, in the case of nano-sized SiO2 not follow a linear relationship with filler weight, which was observed for micron-sized fillers. The nonlinear relationship between filler weight and o-Ps yield could be due to out-diffusion of positrons and/or o-Ps from the filler particles to the matrix. (C) 2004 Elsevier Ltd. All rights reserved.