Coupled-cluster calculations with extended basis sets that include noniterative connected triple excitations (CCSD(T)) have been used to study the FOOOF isomer of F2O3. Second-order Moller-Plessett perturbation theory (MP2) and density-functional theory (B3LYP functional) calculations have also been performed for comparison. Two local minima of similar energy, namely, conformers of C-2 and C-s symmetry have been located. Structures, harmonic vibrational frequencies, and standard enthalpies and free energies of formation have been calculated. The calculated bond lengths of F2O3 are more characteristic of those in F2O and a "normal" peroxide than the unusual bond lengths in F2O2. Both conformers have equal F-O and O-O bond lengths, contrary to a recent suggestion of an unsymmetrical structure. The harmonic vibrational frequencies can aid possible identification of gaseous F2O3. The calculated Delta H-f degrees and Delta(f)G degrees are 110 and 173 kJ mol(-1) respectively. These values are based on extrapolation of CCSD(T) results with augmented triple- and quadruple-zeta basis sets and are expected to be within chemical accuracy (i.e., 1 kcal mol(-1) or 4 kJ mol(-1)). F2O3 is calculated to be stable to decomposition to either FO + FOO or F-2 + O-3, but unstable to decomposition to its elements, to F2O2 + 1/2O(2), and to F2O + O-2.