The vibration-rotation spectra of both 35Cl and 37Cl isotopomers of HOCl have been recorded in selected regions between 3900 and 10 500 cm−1More than 6000 transitions were assigned to the following overtone and combination bands: ν1+ ν3, ν1+ ν2, ν1+ 2ν2, and 3ν1. A Watson-type Hamiltonian in the Sreduction and Irrepresentation was adopted for the analysis, and the vibration-rotation parameters were derived for the excited states. Their values, together with spectroscopic parameters of the fundamental bands reported in the literature, allowed the calculation of a large set of anharmonicity constants and the vibrational dependence of the rotational constants.