Acoustic attenuation in a mixture of gases results from the combined effects of molecular relaxation and the classical mechanisms of viscosity and heat conduction. As a result, the attenuation depends on the composition of the gas, frequency, temperature, and pressure. A model of the relaxational attenuation that permits the prediction of acoustic attenuation is used to predict the effect of composition, frequency, temperature, and pressure on the acoustic attenuation in a three-component gas mixture of nitrogen, methane, and water vapor. The attenuation spectrum is dependent upon the composition through the appearance of peaks in the spectrum related to the relaxation frequency of the particular components and their relaxing complexes. The relaxation peak related to methane dominates the spectrum except at low methane concentrations, where the nitrogen peak, which is dependent upon the water vapor concentration, is evident. Temperature and pressure significantly alter the relaxation frequency and the degree of attenuation, but water vapor plays little role in the attenuation at moderate and high methane concentrations. [This work was funded by the Department of Energy under subcontract from Commercial Electronics, Inc. (Broken Arrow, OK) and by the Ford Motor Company.]