Coextrusion through a series of multiplying die elements enables the production of polymeric films containing thousands of layers of alternating polymers with individual layer thicknesses below 100 nm [1]. Previous studies have shown improved macroscopic properties (especially permeability) related to the nanoconfinement of the polymers within the layers [2], [3]. However, when a lower limit in thickness is reached, breaks and instabilities during process may occur and thus affect integrity and homogeneity of layers. In this study, films with 1025 to more than 8000 alternating layers of PMMA and PS, mass ratio of 50/50, 90/10 and 95/5 wt% and different draw ratios were fabricated in order to identify the materials and processing parameters responsible for these instabilities. Films were characterized by atomic force microscopy (AFM) and a statistical analysis was used to determine the distribution in layer thicknesses and the stability of layers. Results demonstrate the existence of a critical thickness around 20 nm for the PS/PMMA multilayer system, below which the layers break up. This critical thickness is reached whatever the processing parameters studied, suggesting it might be a physical limit due to macromolecular parameters. This work, through ISOCEL project, is supported by the French government agency ADEME and Région Aquitaine. [1] W. J. Schrenk, 3,884,606, 20-May-1975 [2] M. Ponting, A. Hiltner, E. Baer, Macromol. Symp. 2010, 294, 19-32 [3] H. Wang, J. K. Keum, A. Hiltner, E. Baer, Macromolecules 2009, 42, 7055-7066