Prolonged exposure of the hydrogen form of a cation-exchange resin—a sulfonated copolymer of polystyrene crosslinked with divinylbenzene—to gamma radiation and flowing water caused more drastic changes in the chemical and physical properties of the material than has been reported by other investigators for resin exposed to like dosage in a static system. After a dose of 0.75 × 109 rads in a dynamic system, the rate of loss of strong-acid capacity was 20 to 25%/(W-h g) of dry resin, compared with the 4% and the 10 to 20% found by others for the static system. Also, de-crosslinking of more than 4% of the resin matrix accompanied this loss of capacity, compared with the more moderate de-crosslinking or even additional crosslinking reported for the static system. Gamma radiation also caused gas evolution, bead swelling, and produced a weak-acid capacity in the resin equivalent to 3 to 5% of the original strong-acid capacity. Decomposition products included soluble sulfuric, sulfonic, and oxalic acids and insoluble bits of resin. The average rate of loss of sulfur during exposure was estimated to represent 1.0 to 1.2 atoms lost per 100 eV of energy absorbed.