Polyethylene and four samples of chloropolyethylenes of different chlorine content were studied with use of positron annihilation spectroscopy. In positron annihilation lifetime spectra measured for these, two long-lived components occurred. The longest-lived was ascribed to ortho-positronium trapped and annihilated by pick-off in free volume of samples. Changes in intensity and the lifetime of the component reveal the influence of chlorine on microstructure of samples. On the basis of the positron lifetime spectra distributions of free-volume radius were reconstructed for the studied samples. Both the concentration and the average size of free-volume holes decrease with increasing chlorine content. This proves that the reactivity of samples, dependent on the free volume surrounding macromolecules and the arrangement of chains, is influenced by chlorination. It may be very useful to bear in mind the influence of the chlorine on the free volume characteristics when samples of well-defined viscoelastic properties are to be synthesized.