Assume a two-stream problem having one portion of the boundary the fuel stream with fuel mass fraction  and another portion of the boundary the oxidizer stream with oxidizer mass fraction . For example, if the oxidizer stream is air and the fuel stream contains only the fuel, then  and . In addition, assume there is no oxygen in the fuel stream and there is no fuel in the oxidizer stream. Let  be the mass of oxygen required to burn unit mass of fuel (for hydrogen gas,  and for  alkanes, ). Introduce the scaled mass fractions as  and . Then the mixture fraction is defined as
where 
is the stoichiometry parameter, also known as the overall equivalence ratio. On the fuel-stream boundary,  and  since there is no oxygen in the fuel stream, and hence . Similarly, on the oxidizer-stream boundary,  and  so that . Anywhere else in the mixing domain, . The mixture fraction is a function of both the spatial coordinates  and the time , i.e., 
Within the mixing domain, there are level surfaces where fuel and oxygen are found to be mixed in stoichiometric proportion. This surface is special in combustion because this is where a diffusion flame resides. Constant level of this surface is identified from the equation , where  is called as the stoichiometric mixture fraction which is obtained by setting  (since if they were react to consume fuel and oxygen, only on the stoichiometric locations both fuel and oxygen will be consumed completely) in the definition of  to obtain