Modulation frequency dependences of photoacoustic (PA) signal intensity and phase in a transmission geometry were experimentally measured and theoretically calculated for CdInGaS4. It was shown that the PA signal originated from both the electronic transport process and the heat diffusion process in the sample, which could be separated at a characteristic frequency f(min). It was found that the PA signal phase reversed and the PA signal intensity showed a minimum at the frequency f(min).