Opto–acoustic detectors are widely used in laser gas analysis and spectroscopy. As a rule, these detectors are employed under atmospheric and, sometimes, under lower pressures. In this case the pressure dependence of opto–acoustic response appears. This paper describes the results of study of such dependences for H2O, CO2, and their mixtures with different buffer gases with CO2–laser radiation at λ = 10.6 μm. It was very interesting to compare the changes of photoacoustic amplitudes with pressure in these two gases because of specific behaviors of their absorption coefficients.