Two types of composites, high-density polyethylene (HDPE)/Bentonite (BT) and high-density polyethylene grafted with acrylic acid (HDPE-g-AA)/(BT), are prepared by melt compounding. The microstructure of the composite has been studied by the means of positron annihilation lifetime spectroscopy (PALS). It has been found that the mean free volume size is nearly the same in composites and HDPE matrices with different BT concentration. While the ortho-positronium (o-Ps) intensity decreased for HDPE-g-AA and its lifetime distribution is narrower than that for pure HDPE. With the increasing of BT content, the o-Ps intensity increases for HDPE-g-AA/BT cornposites and the o-Ps intensity decreases for HDPE/BT composites. It is found that the carboxyl group of Acrylic acid plays a significant chemical inhibition on positronium formation. (c) 2008 Wiley Periodicals, Inc.