An apparatus for an accurate, repeated measurement of the acoustic resonance frequencies of a photoacoustic cell is described. The shift of the resonance frequency of the first radial mode, caused by mixing of CH4and N2in the resonator, is monitored by repeated recording of the acoustic resonance curves. The time needed to reach the equilibrium concentrations varies between minutes and several days depending on the mixing procedure employed and the composition of the gas mixture. It is shown that condensation of the gases for a short time accelerates the mixing process considerably.