The theory of generation of the second harmonic of a nonlinear photoacoustic signal by optically inhomogeneous bilayer solid samples is proposed. It is established that for the most interesting cases that occur in the experiment, the amplitude of this harmonic of the PA signal is simply related to the thermal coefficient of the optical absorption coefficient of both layers of the sample. Consequently, measurement of the amplitude of the PA signal allows us to determine the value of the thermal coefficient and, thus, the temperature dependence of the optical absorption coefficient of the system.