This paper presents the far-infrared spectrum and X-ray powder diffraction pattern of the structure I hydrate of cyclopropane at 100 °K, and the powder diffraction pattern of the isostructural ethylene oxide hydrate at 100 °K. Between 360 and 100 cm−1the absorption by cyclopropane hydrate is essentially identical to that by ethylene oxide hydrate, but is shifted to low frequency by about 2%. This shift is undoubtedly related to the hydrogen bonds being slightly longer in cyclopropane hydrate, whose cubic lattice parameter is 11.98 ± 0.02 Å compared to 11.89 ± 0.02 Å for ethylene oxide hydrate, both at 110 ± 20 °K. The absorption by cyclopropane hydrate below 100 cm−1decreases rapidly with decreasing frequency; this confirms that the absorption plateau observed for ethylene oxide hydrate between 100 and about 50 cm−1is due to primarily rotational vibrations of ethylene oxide. A recent statement, that the orientational disorder of the water molecules need not be invoked to explain the far infrared spectrum of ice 1 h, is disputed.