The room-temperature electrical conductivity of High Density Polyethylene-Carbon Black (HDPE-CB) switching composites as a function of the conductive filler (CB) content was studied by Positron Annihilation Lifetime Spectroscopy (PALS). The CB, highly structured and porous, imparted high conductivity to its composite, starting at a low level of loading. Deconvolution of the lifetime spectra into 4 components reflected upon 3 different morphologies of the composite structure. The free volume was probed using o-Ps pick-off annihilation lifetime parameter τ4 as a measure of electron density and the mean free volume cavity radius. The probability of o-Ps formation and the concentration of free volume cavities were probed by the o-Ps pick-off intensity I4. In the blended composite of 29 phr (parts per hundred parts of resin), the mean free volume size increased to 3.56 Å from 3.27 Å in the pristine polymer with the corresponding increase in the free volume to 204.5 Å3 from 146.8 Å3. The relative number of free volume cavities (I4) in the composite remained fewer than the host polymer over the CB concentration studied. The changes in the cavity size and concentration, particularly a decrease in the relative intensity I4 by 10.5% resulted in the introduction of enhanced crystallinity into the composite structure which was evidenced from a variation in crystallinity from %75% in the pristine to 85% in the 29 phr CB-HDPE composite in the X-ray diffraction profiles.