Published kinetic results for the FNO3 system suggest that some reactions of FO2 + NO and FO + NO2 may proceed through an FOONO intermediate. This molecule, an isomer of fluorine nitrate, FONO2, has received no experimental and essentially no theoretical study. The present work employs the quadratic configuration interaction method to investigate the structure, vibrational frequencies, and stability of FOONO. Computations at the QCISD(T)/6-311G(3df)//QCISD(T)/6-311G(2d) level of theory lead to Delta H-f,0(0)(FOONO) = 23.5 +/- 4 kcal mol(-1).