Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of PTFE/silica composites. The positron lifetimes (tau(n)) and intensities (I-n) of PTFE and the composites (30-62% silica) were measured at room temperature as a function of specimen thickness. Four lifetime components were found in PTFE and the composites. The longer lifetime components, tau(3) = 1.4 ns and tau(4) = 4.4 - 4.1 ns, were interpreted to be due to the presence of two different sized free volume cavity distributions within the PTFE/silica composites. A strong silica concentration dependence was found in the bulk intensities (I-3b and I-4b). The I-3b value increased from 13.0% in PTFE to 28.2% in the 62% composite, while the I-4b value decreased from 17.5% in PTFE to 4.5% in the 62% composite. The smaller-void size, free volume fraction (tau(3)I(3b)) values increased linearly between 0 and 100% silica concentration, while the larger void size, free volume fraction (tau(4)I(4b)) values decreased non-linearly with silica concentration. Since silica has a long lifetime component (tau(3) = 1.6 ns), this behavior is ascribed to silica particles occupying the large free volume cavities (370 Angstrom(3)) in the PTFE/silica composites.