The high-resolution infrared spectra of the mono isotopic species F35Cl16O3, F37Cl16O3, F35Cl18O3 and F37Cl18O3 have been analysed in the region of the ν 2 + ν 4 and 2ν 2 + ν 5 perpendicular combination bands, from 1920 to 2060 cm−1. Both bands are perturbed by the anharmonic interaction effective between the v 2 = v 4 = 1 and v 2 = 2, v 5 = 1 excited states of E symmetry. The ν 2 + ν 4 bands are of medium–strong intensity, while the 2ν 2 + ν 5 bands are very weak. Since the origins of the two bands differ by about 10 cm−1, the 2ν 2 + ν 5 bands at lower wavenumbers are almost completely hidden by ν 2 + ν 4. In total, about 3400, 2400, 2800 and 3200 transitions have been assigned for F35Cl16O3, F37Cl16O3, F35Cl18O3 and F37Cl18O3, respectively. The assignments of each dyad have been analysed simultaneously with the inclusion of the anharmonic resonance term in the model Hamiltonian. The ro-vibration parameters of the v 2 = v 4 = 1 and v 2 = 2, v 5 = 1 excited states have been obtained, together with the deperturbed band origins and the anharmonic resonance coefficients W 245. Combining the ν 2 + ν 4 origins with those of the ν 2 and ν 4 fundamentals, the anharmonicity constants x 24 have been derived for the four molecules and compared with the ab initio values calculated taking into account the Fermi resonance.