Adding small amounts of butyl rubber to polyolefin compositions results in a number of performance improvements. Polyethylenes containing butyl show improved ESCR (environmental stress-cracking resistance) and increased resistance to impact, puncture and tear. In polypropylene, butyl provides improved impact strength at low temperatures. In both types, the butyl rubber improves vapour barrier capabilities, chemical resistance and energy absorptivity. Butyl can also be used with mineral fillers to extend polyolefins without reducing their impact strength or flexibility. Present and potential automotive applications for butyl modified plastics include battery boxes; bumper strips; sponge components; tapes; and protective films.The paper illustrates the improvements gained in the properties of polyolefin compositions through the use of butyl rubber, and presents an evaluation of several mineral fillers (alumina, mica, whiting, talc) in polyolefin compositions.