Dilute aqueous solutions of polyethylene oxide, when exposed to γ radiation, form a gel structure. Analysis shows that unlike the situation for polyvinylpyrrolidone solutions, this reaction results primarily from the energy absorbed in the polymer and not indirectly via the radiolytic products of water. The G value for formation of polymer radicals capable of cross-linking equals 4·46. The minimum radiation dose required for gelation is greatly increased when small amounts of thiourea are present. This ʻstabilization protection’ is found to depend markedly on radiation intensity as well as on polymer and additive concentration. Over a wide range of these variables the results conform with those expected for competitive reactions between two polymer radicals ( R ˙) and a polymer radical and additive ( A ). Under steady state conditions the results agree with the equation k1Ic = k2 ( R ˙) 2 + k3 ( R ˙) ( A ), where k3 /√ k2 = 0·2. The mathematical treatment developed here may be applicable in the analysis of other first and second order competitive reactions.