[en] In the radiolysis of N₂O-saturated aqueous solutions the ^·OH radicals in their reactions with polymers give rise to polymer-derived radicals. The kinetics of the formation and decay of these radicals are reviewed. Once a larger number of radicals are formed on one polymer molecule, e.g. upon pulse radiolysis, close-by radicals recombine more rapidly while the more distant ones survive for much longer times. Intermolecular cross-linking (between two polymer chains, increase in molecular weight) and intramolecular cross-linking (formation of small loops, no increase in polymer weight) are competing processes, and their relative yields thus depend on the dose rate and polymer concentration. Hydrogen-transfer reactions within the polymer, e.g. transformation of a secondary radical into a tertiary one, are common and facilitated by the high local density of reactive sites. Due to repulsive forces, the lifetime of radicals of charged polymers is substantially increased. This enables even relatively slow β-fragmentation reactions to become of importance. In the case of poly(methacrylic acid) this even leads to an unzipping, and the situation of equilibrium polymerisation is approached. Heterolytic β-fragmentation is possible when adequate leaving groups are available, e.g. in polynucleotides and DNA. In the presence of O₂, chain scission occurs via oxyl radicals as intermediates