The influence of iodine on the free volume of acrylonitrile-butadiene-styrene (ABS) was investigated by positron annihilation lifetime spectroscopy (PALS). The results indicate the filling of free-volume holes, formation of a positronium-iodine compound (PsI(2)/PsI), and possible charge-transfer complexes (CTCs) in the initial stages and the swelling of iodine in the final stages of sorption. The present study also revealed that iodine acts as a chemical quencher of o-Ps. The average size of the free volume suggests that Ig is the predominant species that fills up the free-volume holes during iodination. The diffusion process in the present case shows non-Fickian behavior and deviates from Fujita＇s free-volume concept as far as the fractional free volume and diffusion coefficient are concerned.