A fixed path ultrasonic interferometer for use in vapors is described, in which the stationary wave system between the quartz source and the reflector is explored by a traveling hot‐wire. The hot‐wire is calibrated for its change of resistance in terms of velocity‐amplitude in the ultrasonic wave system. A correction is made for diffraction of the ultrasonic beam.In this apparatus wavelength and absorption measurements are made in ether, ethylene, benzene, carbon disulphide, and acetaldehyde vapors. All show an absorption considerably greater than the theoretical values based on the Stokes‐Kirchhoff equations.