The influence of vinylidenefluoride (three different compositions, viz. 0, 0.5 and 3.0%) on the free volume of the polymer poly(chlorotrifluoroethylene) has been investigated using the Positron Annihilation Lifetime technique. The Lifetime results indicate that, with the increase in the composition of vinylidenefluoride, the average free volume size of the polymer decreases whereas their number density increases. Further, it has been observed that the glass transition temperature (T-g) of this polymer decreases with the increase in vinylidenefluoride content. The present results complement the literature report that vinylidenefluoride influences mainly the amorphous regions of poly (chlorotrifluoroethylene).