The resources of ultrasonic interferometry have been extended by the development of an interferometer of fixed path. In this device a fixed distance is maintained between a piezoelectric resonator and a reflector. Variation of acoustic reaction of the medium between them is secured by a variation of the acoustic wave‐length produced by changes of temperature, pressure or composition, or in liquids and solids by a change in frequency. It is particularly adaptable to the study of the velocity of sound in gases from the lowest temperatures attainable to approximately 500°C and over a pressure range of at least 100 atmospheres because of the elimination of moving parts and packed joints. The method is also applicable, when results are obtained with two precisely known lengths of path, to the measurement of acoustic absorption and reflection coefficients. Results obtained with air and CO2are given.