There are several interesting results in connection with molecular physics which can be obtained from the coefficients of diffusion of ions into gases. From determinations of these coefficients we are enabled to find the number of molecules in a given volume of a gas, and to compare the charge on an ion in a conducting gas with the charge on a hydrogen ion in a liquid electrolyte. In the present paper the principles which are involved in the theory of the interdiffusion of gases are applied to the diffusion of ions produced in a gas by the action of Röntgen rays. It will simplify matters if we first consider the general theory of the conductivity of gases. Professor Thomson has shown that all the phenomena which are met with can be explained by supposing that the rays produce ions in the gas, the motion of which, when acted on by an electric force, gives rise to the observed conductivity. When the gas has been removed from the influence of the rays the conductivity gradually diminishes, and the disappearance of the ions may be due to three causes, any of which may predominate.