AbstractTwo different reactions have been found to occur simultaneously when graphite is exposed to air which has been ozonized by a high-voltage silent discharge. One is the formation of a lamellar compound with nitrogen pentoxide which is always present in ozonized air. The second reaction is a rapid volatilization because of oxidation, which has also been traced to nitrogen pentoxide rather than to the much less reactive ozone.The lamellar compound has been characterized as an acceptor-type compound in which every two molecules of pentoxide constitute one electron acceptor. Equilibrium concentrations which are established in a few hours in ozonized air amount to about 10wt% of pentoxide at 25°C, and 0.1wt% at 150°C. The oxidation reaction has been studied both in ozonized air and in N2O5. A much slower oxidation occurs in ozonized oxygen which can, however, be considerably accelerated if the graphite is first converted to a lamellar N2O5compound.Pre-irradiation of the graphite causes only minor changes in the rates of compound formation and oxidation.The reactions may constitute hazards to reactors operating at low temperatures.