The pattern of the flow state on a sieve plate can be divided into three regions according to different gas velocities. At the range of lower gas velocity, Region I, the clear liquid layer exists on the floor of the plates and moreover the cellular froth layer is formed on it. In the range of intermediate gas velocity over uG=10 to 15cm/sec, Region II, the cellular froth layer grows up rapidly. In this region, the holdup of the gas phase in the froth layer first increases rapidly with the increase in gas velocity, reaches the maximum value and then decreases. At the range of higher gas velocity, Region III, the cellular structure is broken and the froth creates eddies as it flows. In this region, the holdup of gas phase is independent of gas velocity.In Region I, the liquid-mixing in its direction of flow on the plate obeys diffusion mechanism and is controlled by the mixing in the clear liquid layer. In Region III, the liquid-mixing also obeys diffusion mechanism and is presumed to be caused by the eddies in the froth layer. On the contrary the liquid-mixing in Region II does not always obey diffusion mechanism because the circulation of the cellular froths in the direction of liquid flow is one of the important factors controlling the liquid-mixing.On the other hand, gas phase is presumed to flow through the froth layer in a state close to piston flow in Region III.The conclusions mentioned above and the theoretical relations for the concentration gradient in liquid phase on a tray which were already derived by authors are confirmed by means of the experiments of desorption of ammonia or oxygen in water by air.