The dependence of the diffusion coefficient on temperature for gases can be expressed using Chapman–Enskog theory (predictions accurate on average to about 8%):
where
* D is the diffusion coefficient (cm/s),
* A is approximately  (with Boltzmann constant , and Avogadro constant )
* 1 and 2 index the two kinds of molecules present in the gaseous mixture,
* T is the absolute temperature (K),
* M is the molar mass (g/mol),
* p is the pressure (atm),
*  is the average collision diameter (the values are tabulated page 545) (Å),
* Ω is a temperature-dependent collision integral (the values tabulated for some intermolecular potentials, can be computed from correlations for others, or must be evaluated numerically.) (dimensionless).
The relation 
is obtained when inserting the ideal gas law into the expression obtained directly from Chapman-Enskog theory, which may be written as
where  is the molar density (mol / m) of the gas, and 
with  the universal gas constant. At moderate densities (i.e. densities at which the gas has a non-negligible co-volume, but is still sufficiently dilute to be considered as gas-like rather than liquid-like) this simple relation no longer holds, and one must resort to Revised Enskog Theory. Revised Enskog Theory predicts a diffusion coefficient that decreases somewhat more rapidly with density, and which to a first approximation may be written as
where  is the radial distribution function evaluated at the contact diameter of the particles. For molecules behaving like hard, elastic spheres, this value can be computed from the Carnahan-Starling Equation, while for more realistic intermolecular potentials such as the Mie potential or Lennard-Jones potential, its computation is more complex, and may involve invoking a thermodynamic pertubation theory, such as SAFT.