Generation of acoustic signals in optoacoustic detectors has certain peculiarities at excitation of molecules of an absorbing gas in its cell with a short laser pulse. The main peculiarities are caused by the influence of natural oscillations of the microphone diaphragm. In this study we have shown that at low pressure (p ≤ 10-100 Torr) the signal shape is determined by these natural oscillations. Phase shift of the diaphragm natural oscillations is a function of pressure of a gas under study. We propose to measure this pressure dependence of the phase shift for determining V-T relaxation time of molecular gases. As an example of the feasibility of this approach, we present some experimental results on the relaxation time of the n1 + 3n3 state of H2O molecule in H2O--H2O, H2O--air, and H2O--H2 collisions.