The striation thickness distribution for an impingement mixer is calculated applying the methods of computational fluid dynamics (CFD). The description is based on the three-dimensional flow field equations together with a two-equation turbulence model so that no assumption of the residence time distribution is necessary. A field equation is derived in order to describe the striation thickness where the stretching velocity is calculated using the statistical theory of turbulent diffusion. The distribution of striation thickness follows from the solution of this field equation.