A statistical criterion for the classification of gaseous mixing regimes is presented. It is based on the evaluation of two quantities: the mixing layer thickness and the separation distance between two neighboring interface segments, which are preliminarily defined in a specific section along with other quantities (stretch ratio and stretch rate) relevant to the mixing analysis. A unique experimental method for a lagrangian measure of the aforementioned quantities as a function of the residence time is described. Measurements use a two-dimensional laser light scattering technique in a two-dimensional, transitional, isothermal flow which can be considered a prototypical condition of mixing. The probability density function of the stretch rate as well as average values of stretch ratio, mixing layer thickness and interface separation distance are also reported. The last two quantities are used in the determination of a single parameter named "saturation factor" (Csat) , that allows the identification of three regimes: (a) "Isolated mixing layer regime" (Csat = 1) in which both the mixing and oxidative reactions occur in the neighborhood of the interface. This is a well recognized regime, to which many studies on the characterization of one-dimensional time-dependent stretched diffusion flames refer. (b) "Interacting mixing layer regime" (10 -2 < Csa t < 1) in which the diffusion still occurs in the whole flow field, but more sophisticated models than those relative to the previous regime are needed for its characterization. (c) "Saturated mixing layer regime" (Coa t < 10 -2) in which the mixing process is no longer related to the intermaterial surface in its wholeness, but only to that part for which it is still far from other segments of the interface.