The kinetic theory of gases entails that due to the microscopic reversibility of the gas particles' detailed dynamics, the system must obey the principle of detailed balance. Specifically, the fluctuation-dissipation theorem applies to the Brownian motion (or diffusion) and the drag force, which leads to the Einstein–Smoluchowski equation:where
*  is the diffusion coefficient;
*  is the "mobility", or the ratio of the particle's terminal drift velocity to an applied force, ;
*  is the Boltzmann constant;
*  is the absolute temperature.
Note that the mobility  can be calculated based on the viscosity of the gas; Therefore, the Einstein–Smoluchowski equation also provides a relation between the mass diffusivity and the viscosity of the gas.