The IE for OThF determined in the present work was 0.4 eV higher than the value reported by Lau et al. The previous under-estimation was probably due to the presence of vibrationally excited levels of OThF resulting from the high temperature evaporation method used to obtain gas phase samples. The IE predicted by the B3LYP/DFT calculations, 50375 cm -1 , was in respectable agreement with the measured value of 50678±40 cm -1 .