The　energies　and　structures　of　various　peroxynitric　acid　(HOONO2)　isomers　as　well　as　their　isomerization　and　dissociation　reactions　have　been　investigated　at　the　CBS-QB3　level　of　theory.　One　HOONO2,　five　HOO-ONO,　three　HONO2O,　and　one　HONO3　isomers　were　found　here.　Among　them,　the　HOONO2　configuration　(isomer　1)　is　the　most　stable　one　in　both　the　gas　phase　and　water,　while　the　configuration　HONO3　(isomer　9)　is　the　energetically　highest.　Moreover,　four　HONO　center　dot　center　dot　center　dot　O-2　complexes,　i.e.,　isomers　a,　b,　c,　and　d,　were　found.　Calculated　results　indicate　that　different　isomers　of　HOONO2　can　rearrange　into　each　other　via　one-step　or　multi-step　isomerization.　The　isomerization　and　dissociation　reactions　involving　isomer　1　were　found　to　be　hard　to　occur,　which　implies　that　isomer　1　is　kinetically　stable　in　the　above　reactions.　For　other　isomers,　their　isomerization　is　relatively　easy　to　occur.　Additionally,　the　effects　of　aqueous　solvation　of　water　on　the　isomerization　and　dissociation　reactions　were　also　investigated.　(C)　2010　Elsevier　B.V.　All　rights　reserved.