A high-level ab initio electronic structure study of the unimolecular rearrangement of trans-FONO to fluoryl nitrate (FNO2) is presented. The FO + NO --> [FONO]* --> FNO2 reaction is found to proceed through a tight transition state with a calculated height of 22(3) kcal mol(-1). The isomerization process is described qualitatively in terms of a two-state diabatic model that involves the fluorine atom and the ground and first excited or states of NO2.