Kerr effect measurements allow the determination of the surfactant monolayer rigidity of microemulsion droplets. For a AOT-water-isooctane microemulsion containing water-soluble poly(oxyethylene) in the interior of the water droplets, we find an increase of the monolayer rigidity up to about 1kT. This rigidification is caused by attractive interactions between the polymer and the surfactant which lead to a polymer adsorption at the oil/water interface.