In a gas‐microphone cell absorption of modulated light causes oscillating transport of heat and of mass as well. The transport of mass is described by the diffusion equation. The concentration of substance from sample or sorbate at the sample to gas boundary is obtained from the vapor pressure. Introduction of an average molar concentration variation leads to a pressure variation. Results of calculations can explain recently measured data very well. For water at 40 °C oscillating mass transfer contributes the same amount to pressure variation as the heat wave.