Frequency response tests of two bubble-cap distillation columns are made on methanol-Waater mixture for changes of reflux flow and vapor flow. Sinusoidal response of liquid temperature of each tray is measured by thermocouple and the temperature response is translated into composition response. The frequency response date obtained in this way are compared with the solutions of material balance equations, which are derived under several assumptions and in consideration of the time lag of liquid flow between trays and the transportation lag of reflux pipe line. The experimental data are in fairly good coincidence with the calculated values in the case of reflux flow change. The result reveals that the large phase lag in high frequency range is mainly due to the time lag of liquid flow between trays. The response in low frequency range is well approximated by the first order lag with a time constant common to each tray. Its calculating method is shown in this paper.