AbstractThe tensile behavior of oriented polybutadiene at 83°K is systematically studied as a function of strain rate and pre‐test orientation. Electron spin resonance studies of radical formation are made in conjunction with the mechanical tests. Three different modes of mechanical behavior are observed (brittle, crazing and a second ductile behavior without crazing), depending on test conditions. Radical formation is observed in association with the two ductile modes of behavior. The ESR spectra obtained are attributed to a combination of allyl radicals formed by chain scission between the α‐methylene groups and peroxy radicals. The relative quantity of the two radical species present is thought to be related to the ratio ofcis/trans‐isomerism. Stability of the radicals observed with time and with an increase in temperature is studied. Further studies are made of the quantity of environmental test gases absorbed during