Previous studies of two-layer slot coater have assumed that diffusional and convective mixing of the two miscible liquid layers can be ignored along the interfacial region. In this work, flow visualizations were used to show that diffusional mixing does occur between layers of miscible liquids, as well as convective mixing under certain operating conditions. A series of three dies were investigated which differed in the shape of the separating plate between the two inlet flows. Using square center die blocks, it was found that the presence of the vortices within the coating bead and the position of the separation line depended on the ratio of top layer viscosity to bottom layer viscosity. A knife-shaped center block caused several transients in the flow within the gap. A rounded or bullet-shaped center block resulted in the elimination of the vortex that caused convective mixing of the two liquid layers for a viscosity ratio of 10 : 1. Experiments showed good agreement with a two-dimensional Newtonian model that calculated variations in liquid properties across the interfacial region by defining viscosity as a function of a tracer species concentration, where the isoconcentration contours were then used to define the interfacial region.