The structure, stability, interconversion, and unimolecular decomposition processes of gaseous protonated nitrosyl fluoride, ONF, have been investigated by mass-analyzed ion kinetic energy (MIKE) and collisionally activated dissociation (CAD) mass spectrometry, as well as ab initio GAUSSIAN-1 calculations. Positive evidence has been obtained for the existence of two distinct, not easily interconvertible isomers, HO-NF+, 1, and ON-FH+, 3. These two species are distinguishable by both CAD and MIKE spectrometry, whose results are consistent with the theoretical description of the system. The fluorine-protonated isomer 3 is the global minimum on the surface and is more stable than the oxygen-protonated isomer 1 by 70.1 kcal mol(-1). Whereas isomer 3 has a low-energy dissociation channel to produce NO+ and HF, the less stable isomer 1 is trapped in a deep potential well, which prevents both rapid dissociation and isomerization to 3. The GAUSSIAN-1 potential energy diagram explains the distinctly different shapes of the MIKE peaks and the kinetic energy releases (KERs) associated with the reaction (ONFH+ --> NO+ + HF. The loss of HF from 3 gives rise to a narrow peak with a small KER, while the same reaction from 1 is associated with a dish-topped peak and an exceptionally large KER (E(t1/2) = 2.68 eV).