From equilibrium mixtures of cis- and trans-FC(O)OF, complete sets of the nine fundamentals and several combinations were measured by gas phase IR, matrix IR and low-temperature Raman spectroscopy. Assignment of bands to the cis or trans conformer was achieved by gas band contour analysis and by comparison between observed and ab initio calculated (MP2/6-31G*) vibrational spectra : The enthalpy difference between cis- and trans-FC(O)OF amounts to Delta H degrees = H degrees(cis) - H degrees(trans) = 4.9(3) kJ mol(-1) derived from the matrix infrared bands of thermal effusive molecular beams at different temperatures quenched as argon matrices. The respective entropy difference amounts to 1.45 J mol(-1) K-1 as calculated from statistical thermodynamics. A normal-coordinate analysis was performed for both conformers using their fundamental wavenumbers of different isotopomers. The molecular structures of both conformers at 298 K (14% cis-, 86% trans-FC(O)OF) were determined by gas electron diffraction and theoretical calculations. Ab initio calculations at the HF and MP2 levels overestimate the energy difference between the two conformers whereas local and nonlocal density functional theory calculations predict a reversed conformational stability. The bonding properties of cis- and trans-FC(O)OF are discussed in terms of their force constants and bond lengths.