Alpha particles crosslink films of polyethylene more effectively than do sparsely ionizing radiations, but even in the absence of oxygen [alpha]-rays produce simultaneously some main-chain scission. Polymethyl methacrylate films were degraded by [alpha]-particles, but the energy needed to produce a main-chain break (detected by viscosity measurements) was 263 ev as opposed to approximately 60 ev with 2- Mev electrons. Addition of naphthalene and 8-hydroxyquinoline to the polymethyl methacrylate did not protect against [alpha]-radiation, although these substances did protect against sparsely ionizing radiations. The reasons the relative effectiveness of [alpha]-rays is greater than that of sparsely ionizing radiations for crosslink-ing and less for degradation are discussed. It is postulated that the molecules in polymethyl methacrylate are in a randomly coiled configuration and are broken at several points where a segment is traversed by an [alpha]-particle, the small molecules produced by the closely spaced breaks being without effect on the viscosity molecular weight. In polyethylene films the molecules are oriented parallel to the sheet, and the path length of an a-particle through them is such that few ionizations are wasted. The production of main-chain scission is attributed to dissociation of a C[long dash]C bond. Recombination by a Franck-Rabinowitch "cage" effect is postulated to occur with sparsely ionizing but not with densely ionizing radiations.