Poly(ethylene oxide) was irradiated with pulses of 16 MeV electrons (pulse length up to 100 ns) in aqueous and acetonitrile solutions in the absence and presence of oxygen. The change of the light scattering intensity (LSI) after the pulse was monitored. In acetonitrile solution, at a concentration of 2,3 · 10−2 base mol/1, main chain rupture occurred both in the presence and in the absence of O2. In aqueous solutions at [PEO] = 2,3 · 10−2 base mol/1 the LSI increased after the pulse in the absence of O2 (crosslinking) and decreased at [O2] = 1,4. 10−3 mol/1 (main chain scission). The rate of LSI decrease was correlated with a diffusion process in the case of O2 free acetonitrile solution and with a chemical reaction in the cases of O2 saturated aqueous and acetonitrile solutions. In the latter two cases it was found that a bimolecular reaction precedes bond scission in the main chain. It is assumed that peroxyl macroradicals (POmath image) react with each other forming oxyl macroradicals (PO) which decompose relatively rapidly by main chain scission.