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The chemical relaxation of the N2O4 dissociation was studied using Ar+ laser excited acoustic resonances of a cylindrical cavity. The resonance profiles of the first radial mode were measured as a function of pressure and temperature by a computer controlled setup. The N2O4 dissociation rate constant and the mean rotational and vibrational relaxation time of the N2O4–NO2 mixture were obtained for temperatures between 273 and 317 K by fitting a detailed theoretical model of the resonator to the measured values for frequency dispersion, resonance broadening, and strength of the photoacoustic signal. Excellent agreement between theory and experiment was obtained for the frequency dispersion and the photoacoustic signal. An empirical correction function had to be introduced to account for discrepancies between the theoretically predicted and measured values of the resonance broadening which was in the percent region. A value of 6.1×10−15 cm3/molecule s was determined for the rate constant of N2O4 dissociation in the low pressure limit at 298 K. The activation energy of the N2O4 dissociation for 20 mbar pressure was determined to be EA=36 kJ/mol. A comparison of the measured low-pressure limit of the unimolecular rate constant with theoretical predictions of RRKM theory yielded values for the mean collision efficiency of N2O4 and NO2 between 0.95 and 0.5 in the temperature range 273–317 K.