Abstract The millimeter wave rotational spectra of the four isotopic species of indium monochloride have been studied in the 250–300 GHz frequency region. The molecules were produced by the reaction of indium heated at 1400°C and CCl 4 inside a free space cell. The analysis of the spectra led to improved and extended sets of Dunham parameters for the isotopic species 115In 35Cl, 115In 37Cl, 113In 35Cl and 113In 37Cl. From these data the vibrational constants and Dunham potential constants are derived. Comparison of the most precisely determined Dunham parameters Y 01 for the isotopic pairs of In 35Cl and In 37Cl shows deviations from the general mass relations between isotopic species, indicating a violation of the Born-Oppenheimer approximation.