High-resolution Fourier Transform Infrared spectroscopy (FTIR) spectra of ethylene oxide (CH_2_-O-CH_2_) have been measured in the far-infrared region using synchrotron radiation. A total of 1182 lines between 15/cm (450GHz) and 73/cm (2.2THz) were assigned, with J_max_=64, expanding upon previous studies that had recorded spectra up to 12/cm, J_max_=49. All available data were co-fitted to provide greatly improved rotational constants for the ground vibrational state that are capable of predicting transitions up to 73/cm.