The high-resolution infrared spectra of the monoisotopic species F 35Cl 16O 3, F 37Cl 16O 3, F 35Cl 18O 3 and F 37Cl 18O 3 have been studied in the region of the 2ν 5 overtones, from 1100 to 1200 cm −1. Both the parallel 2 ν 5 0 and the perpendicular 2 ν 5 ∓ 2 components are clearly observed in the spectra, their origins differing by about 0.4 cm −1. In each spectrum about 2000 transitions have been assigned, 35% of them belonging to 2 ν 5 0 . The parallel and perpendicular bands in each manifold have been analyzed separately since no evidence of perturbations has been observed. The rovibration parameters of the v 5 = 2, l 5 = 0 and v 5 = 2, l 5 = ∓ 2 excited states have been obtained. For the four species combining the 2 ν 5 0 and 2 ν 5 ∓ 2 band origins with those of the ν 5 fundamentals the harmonic wavenumbers, ω 5 0 , and the x 55 and g 55 anharmonicity constants have also been derived.