A Hartree–Fock and post-Hartree–Fock study on thecis–transequilibrium of fluoroformyl hypofluorite, FC(O)OF, is presented. Pople's split-valence double- and triple-zeta quality basis sets, augmented with sets of polarization and diffuse functions (6-31G(d), 6-311G(d), 6-311G(2df) and 6-311+G(2df)), correlation consistent polarized valence triple-zeta Dunning's (aug-cc-pVTZ), and medium-size polarized Sadlej's (MSPBS) basis sets were used at the HF and at the MP2, MP4(SDQ), MP4(SDTQ), CCSD, CCSD(T), and DFT correlated levels. For all applied methods,trans-FC(O)OF is predicted to be more stable than thecisisomeric form, in agreement with experimental data. ΔE(cis⇒trans) values range from – 1•70(CCSD/6-311G(d)) to –0•16 kcal mo1-1(DFT/6-311G(d)) and thecis–transbarrier height ranges from 9•60 (MP2/6-311G (2df)) to 8•56 kcal mo1-1(MP4(SDQ)/6-311+G(2df)). Reliable geometries and IR harmonic vibrational frequencies were obtained using the DFT model with the Becke–Lee–Yang–Parr exchange-correlation potential.