A study of the effect of microphone design on sound absorption and velocity measurements has been carried out for pressures ranging from 1 to 18 atm. Results of this research show clearly that losses may be introduced by the microphone itself and that this problem becomes more severe with increasing pressure. A microphone has been developed which completely eliminates such effects at lower pressures and greatly reduces them at higher pressures. Measurements inCH4gave a pressure dependence of the effective specific‐heat ratio in good agreement with theory. Extrapolation to zero pressure gave a result in good agreement with that obtained by using the vibrational specific heat calculated from the Planck‐Einstein formula. Relaxation times τvwere determined as a function of pressurep0and lead to the averaged valuep0τv = 1.565±0.013 (μsec⋅atm). This is in good agreement with recent results using the spectrophone method (1.60 μsec⋅atm) and in significant disagreement with results obtained from fluorescence experiments (1.90 μsec⋅atm). Data obtained forO2gave relaxation timesτv = 3.34(−2) sec⋅atmat room temperature andτv = 2.36(−2) sec⋅atmat a temperature of approximately 50°C. The room‐temperature value is higher than previously published data by a factor of 2 and is in reasonably good agreement with extrapolation of high‐temperature shock‐tube data. The value obtained at 50°C is in still better agreement with this extrapolation.