Polyisobutene (PIB) dissolved in saturated hydrocarbons was degraded by irradiation with 2 ,us impulses of 15 MeV electrons. The time dependence of the change of light-scattering intensity (LSI) (due to the decrease of molecular weight) was measured. The half life r I,_,(LS) of the decrease of LSI was not influenced by the addition of cyclohexene, which on the other hand drastically reduced the extent of degradation, leading to the conclusion that the time of establishing bond scissions in the main chains is much shorter than rx;,(LS ) and the latter corresponds to disentanglement diffusion. This is furthermore evidenced by the following experimental findings: r L _,(LSt increases with decreasing PIB concentration, i.e. with decreasing macroviscosity. However, T~ _,(LS) increases linearly with increasing microviscosity as shown by measuring T~,2(LS ) in different hydrocarbon solutions 1('~ C~,) r~ e(LS) is proportional to M°3'k i.e. significantly weaker dependence on N-' I than expected for translational diffusion. Thus, it is believed that disentanglement diffusion and translational diffusion may be distinguished As n-propanol was successively added to 17-hexane solutions of PIB. r~ _,ILSI increased, indicating that disentanglement diffusion is easier for Ioosel~ lhan for lighll.~ coiled macromoleculcs.