AbstractThe 2,4‐dinitrophenylhydrazones formed on irradiated polystyrene films were regenerated to carbonyl groups by immersing the films in levulinic acid solutions. The changes in the amount of carbonyl groups regenerated from the hydrazones were inferred by comparing the absorptions of the hydrazones at 378 mμ. The carbonyl groups were regenerated from the hydrazones with increase in regeneration times in the early stages of the regenerations and gradualy regenerated with increase in the regeneration times thereafter. The regeneration of the carbonyl groups was facilitated by raising the temperature in the levulinic acid solution, by adding dilute hydrochloric acid to the levulinic acid, or by raising the temperature in the levulinic acid solution containing hydrochloric acid. When the polystyrene films were irradiated for different periods of time, the increase in the critical surface tension of the films with increase in irradiation time showed a trend similar to the increase of amount of hydrazones formed on the surface zones with increase in irradiation ti