The structure of the subsurface layer of polymethylmethacrylate (PMMA) formed by bombardment with low-energy ions of Ar is reported. It was found that the subsurface region contains a graphitized, cross-linked, and low-molecular weight layers. We argue that ion etching of PMMA is mostly determined by the properties of the top graphitized layer and the processes leading to the formation of this layer. Also, it was found that ion etching causes various defects and typical features to appear on the surface of PMMA: bubbles, waves, and a net with a cell of nanometer size. The stratification of PMMA was demonstrated to play an important role for the development of the surface topology. (C) 2004 American Vacuum Society.