The microwave spectrum of ethyl hypochlorite has been analyzed in detail in the region of 20-60 GHz. Observed transitions for C 2H 5O 35Cl in the ground state have been fit to a Hamiltonian model which includes p4 centrifugal distortion terms. The lowest vibrationally excited state of C 2H 5O 35Cl and the ground state and lowest vibrationally excited state of C 2H 5O 37Cl were analyzed with a rigid rotor model. This lowest vibrational mode lies at 125 ± 23 cm -1 and is most likely the torsional motion about the CO bond. The dipole moment has been measured and found to have two nonzero components; μa = 1.623 ± 0.010 D, μb = 1.097 ± 0.005 D. No A-E torsional splittings were observed in either the ground state or the v = 1 state implying a lower limit for the barrier to internal rotation of ∼3.0 kcal/mole. Ethyl hypochlorite was synthesized in the waveguide by the reaction of chlorine nitrate with ethanol.