A general valence force field for FONO2 is deriverd from Ne-matrix infrared spectra together with N-15 and O-18 isotopic shifts. A gas electron diffraction study results in a planar structure with the following geometric parameters (r(a) distances and L(a) angles with 3 sigma uncertainties) : N=O = 1.184(2) Angstrom, N-O = 1.507(4) Angstrom, O-F = 1.409(5) Angstrom, O-N=O-c = 117.1(9)degrees, O-N=O-t = 108.4(18)degrees, O-c=N=O-t = 134.5(21)degrees, and N-O-F = 106.0(11)degrees. O(c)and O-t, are the oxygen atoms cis and trans to the O-F bond. The N-O single bond in fluorine nitrate is ca. 0.1 Angstrom longer than those in nitric acid and methyl nitrate. Various ab initio calculations predict rather different N-O bond distances between 1.384 and 1.609 Angstrom, depending on the level of theory (HF, MP, CC, or CI) Local density functional theory reproduces the experimental geometry perfectly. Nonlocal corrections, however,make the results drastically worse.