Thermal and pressure activation parameters are reported for the decomposition of peroxynitrate, isomerization of peroxynitrite, and their reduction by iodide in aqueous solutions, The spontaneous decomposition reactions are characterized by activation enthalpies of ca. 16 kcal/mol, activation entropies close to zero, and significantly positive activation volumes between 7 and 10 cm(3)/mol, These parameters suggest that the rearrangement of both peroxo species, leading to intermediates of increased partial molar volume, must include partial bond cleavage. The iodide-induced reduction reactions are characterized by significantly smaller activation enthalpies, negative activation entropies between -19 and -23 cal/(mol . K), and negative activation volumes between -6 and -11 cm(3)/mol. These parameters suggest that bond formation between the redox partners occurs before electron transfer and favors an inner-sphere mechanism. The results are discussed regarding mechanisms based on ambient kinetic data suggested in the literature.