In the infrared spectrum of gaseous ethylene oxide the A1 C—H stretching and CH2 wagging modes are assigned at 3018 and 1148 cm−1, respectively. The previous assignment of the B1 ring deformation mode at about 890 cm−1 is not consistent with the spectrum, but its correct assignment is not clear. The infrared spectrum of ethylene oxide liquid and the frequencies of the fundamental modes in the gas and liquid states are presented.