Ethylene propylene elastomers (EPDM) processing and physical property performance depend on molecular structure. Composition, molecular weight and molecular weight distribution are all controlled to give optimum properties for specific applications. For many processing opera ions and end-use physical properties very important structural variables are long chain branching (LCB) and gel . These two variables alter bulk rheological properties thereby strongly influencing cold flow, mixing performance, extrusion characteristics and vulcanizate network perfection. Long chain branching is sometimes intentionally introduced to improve pr ocessing properties. This paper discusses (Slide 1) the mechanisms for LCB and gel formation and methods for their determination.