A fixed path interferometer, using variable temperature for the measurement of the absorption in gases in the lowf/pregion is described. Its use is illustrated by measurements on argon, neon, air, nitrogen, and nitrogen‐hydrogen mixtures at 250 kc. The values found for argon and neon were above classical by 40 and 65 percent, respectively. The discrepancies are ascribed to Rayleigh cross modes in the ultrasonic waves excited by the quartz crystal transducer rather than to any departure from classical theory. Cross modes and their influence on ultrasonic measurements are discussed. Data on the performance of two crystals at 250 and 500 kc are given.