Darken's first equation is:
where:
*  is the marker velocity of inert markers showing the diffusive flux.
*  and  are the diffusion coefficients of the two components.
*  and  are the atomic fractions of the two components.
*  represents the direction in which the diffusion is measured.
It is important to note that this equation only holds in situations where the total concentration remains constant. 
Darken's second equation is:
where:
*  is the activity coefficient of the first component.
*  is the overall diffusivity of the binary solution.