Nanolayer coextrusion is an innovative process allowing to create films composed of thousands of layers of alternating polymers with individual layer thicknesses below 100 nm [1]. It has been observed that below a certain layer thickness, the layers tend to lose their integrity, i.e. they break spontaneously during the process. This study focuses on polystyrene (PS) / poly(methyl methacrylate) (PMMA) nanolayered films: different films have been fabricated with the aim to reach individual layer thicknesses as small as possible, varying the number of layers, the mass composition of both components and the final total thickness of the film. Films are characterized by atomic force microscopy (AFM) and a statistical analysis is used to determine the distribution in layer thicknesses and the continuity of layers [2]. For the PS/PMMA nanolayered systems, results point out the existence of a critical layer thickness around 10 nm, below which the layers break up. Possible mechanisms for critical layer thickness are examined and discussed. We propose this beak up phenomenon is due to small interfacial perturbations that are amplified by (van der Waals) disjoining forces.