(1) The initial rate of solution of gas into liquid was developed from a new theory of the rate of solution based upon the assumptions. (a) that among the molecules of gas, which collide with the interface, only those, whose components of velocities at right angles to the interface are greater than a threshold value u0, can enter into the liquid phase, and (b) that among the molecules of gas in the liquid phase, only those, which collide with the interface with velocities, whose components at right angles to the interface are greater than a threshold value u0′ can escape from the liquid phase. The equations derived from the new theory were compared with those obtained from the diffusion layer theory. (2) The solubility of gas was expressed as a function of u0 and u0′. (3) The values of u0 and u0′ were calculated in the case of oxygen and water at 15°C. (4) Henry’s law was derived by means of the new theory. (5) The distribution of a substance between two liquid phases was discussed in exactly the same manner.