An investigation of the interface in a phase-separated colloid-polymer suspension with ellipsometry is presented. The coefficient of ellipticity at the Brewster angle of light reflected at the interface between the coexisting colloid-rich and polymer-rich phase was measured along two trajectories through the phase diagram. Interpreting the data in terms of a diffuse interface model, an interfacial thickness in the order of the diameter of the colloidal particles is found.