In turbulent reacting flows of practical interest, the application of mixture fraction based combustion models is computationally burdensome due to the added dimension of the mixture fraction. (The term "mixture fraction based combustion model" refers to an approach that uses the mixture fraction as an independent variable.) Recently, Buriko et al. [1] have suggested that the quasi-steady approximation in mixture fraction based modeling is applicable to predict average species mass fractions in turbulent hydrogen/air jet flames. This holds potentially great advantages in treating more general, nonpremixed, reacting Rows which may be computationally intractable without resorting to quasi-steady modeling. This paper extends the quasi-steady modeling investigation of [1] to conditionally averaged quantities. The validity condition for quasi-steady (mixture fraction based) modeling is derived and discussed. As practical illustration, the condition is applied to the turbulent hydrogen/air jet diffusion flames of Barlow and Carter [2-4].