The effect of stress (hydrostatic compression) on the free volume of poly(chlorotrifluoroethylene) has been investigated using the positron annihilation lifetime (PAL) technique in the pressure range of 0-14 kbar at room temperature. The variation of the annihilation parameters indicates that the free volume compression occurs at a higher pressure (>8 kbar) compared to molecular compression (4 kbar). The measured free volume size decreases from 95 to 73 Angstrom(3) under the influence of compression. The results also indicate a crystalline-amorphous transformation. The free volume compressibility (beta(f)) is found to be maximum around 9 kbar (10(-3) atm(-1)).