The ultrasonic resonator interferometer has been used to measure the ultrasonic velocity at 30°C in 1,1‐dichloroethane (ethylidene chloride,CH3CHCl2) and 1,2‐dichloroethane (ethylene chloride,CH2Cl‐CH2Cl) vapors as a function of frequency over pressure. The rangesf/pemployed are 0.8 to 100 Mc sec−1atmos−1in the case of ethylidene chloride and 1.0 to 100 Mc sec−1atmos−1in the case of ethylene chloride. The velocities have been corrected for nonideality of the two vapors. Dispersion of ultrasonic velocity occurs in both the cases and this is ascribed to the slow rate of energy exchange between translational and vibrational states. There is evidence of double relaxation in the two cases, the principalf/pratios at 30°C being 76.5 Mc sec−1atmos−1for the former and 102 Mc sec−1atmos−1for the latter.