The thermal decomposition of dehydrated nickel oxalate is accelerated by preirradiation with γ-rays (about 1.1 Mev). The acceleration is preceded by an initial deceleratory reaction that obeys the unimolecular decay law. The pressure of gas liberated in this reaction is proportional to the γ-ray dose. It is suggested that preirradiation produces nickel and an unstable intermediate which decomposes on heating. During the acceleratory and decay periods decomposition occurs by the Prout-Tompkins mechanism.