The high resolution infrared spectrum of the mono-isotopic species F 35 Cl 16 O 3 has been studied in the region of the 2ν 4 overtone, from 2560 to 2680cm −1 . The 2ν4∓2 perpendicular component is strong and clearly observed while the 2ν40 parallel component is very weak and almost completely hidden by the perpendicular one. Their origins differ by 12.6cm −1 , the 2ν4∓2 being located at higher wavenumbers. The 2ν4∓2 band is perturbed by the anharmonic interaction between the v 4 =2, l 4 = ∓2 and v 2 =v 4 =v 5 =1, l 4 =l 5 = ±1 excited states, both of E symmetry. In total 3157 transitions have been assigned, 83% of these to 2ν4∓2, 12% to ν2+ν4±1+ν5±1, and 5% to 2ν40. The three bands have been analyzed simultaneously, taking into account the Fermi resonance effective between the excited states of E symmetry. The ro-vibration parameters of the excited states have been obtained, including the deperturbed band origins of 2ν4∓2 and ν2+ν4±1+ν5±1, at 2628.5890(4) and 2619.3342(5) cm −1 , respectively. The W245 anharmonic constant is equal to 4.0161(4) cm −1 . The x44+g44 and x24+x45+g45 anharmonicity constants have been derived from the obtained band origins and those of ν 4 and ν 2 +ν 5 .