The thermal decomposition of p-nitroperoxybenzoic acid has been studied both in a static system in vacuo and in the presence of product gases in the range 90–105°. p-Nitrobenzoic acid, carbon dioxide, water, and nitrogen are the principal products. The stoicheiometry of the reaction conforms to the equation 25 NO₂·C₆H₄·CO·O·OH → 23 NO₂·C₆H₄·CO₂H + 14 CO₂+ 5 H₂O + N₂ The reaction is of the gas–solid type and its course can be represented by the expression, α=ktⁿ, where α is the fraction of p-nitroperoxybenzoic acid decomposed in time t, k is a rate constant, and n a constant. In the range α= 0·05–0·9, n is approximately 1·5. The activation energy for the process is 38 ± 3 kcal./mole. A reaction mechanism has been proposed which involves the rupture of the peroxy-link followed by a rapid reaction of hydroxyl radicals. The initial reaction is catalysed by p-nitrobenzoic acid and other substituted benzoic acids with high melting points. Oxalic acid inhibits the initial step but is preferentially attacked by hydroxyl radicals.