A resonant tube has been developed to measure sound absorption in gases, with specific emphasis on the vibrational relaxation peak of N2, over a range of frequency/pressure ratios from 0.1 to 2500 Hz/atm. The experimental background losses, measured in argon, show good agreement with the theoretical wall losses except at a few isolated frequencies. Measures to minimize spurious contributions to the background losses include the use of rigid cavity terminations, external excitation, and a differential technique of background evaluation. Room temperature measurements of sound absorption in binary mixtures of N2–CO2, in which both components are excitable, yield the following results for the locations of the respective vibrational relaxation peaks in Hz/atm: (f/P)max=0.063 + 123mfor the N2peak,m=mole percent of added CO2; (f/P)max=34 500−268mfor the CO2peak,m=mole percent of added N2. The figure 0.063 is an upper bound due to the presence of 2.5 ppm H2O.