Polystyrene was modified by low energy hydrogen ions in ultrahigh vacuum. The modified surfaces were analyzed in situ by x-photoelectron spectroscopy and ex situ by atomic force microscopy. Results revealed that 10 eV H+ ion bombardment on polystyrene produces a cross-linked network film, while the polymer backbone remains intact and undesirable impurity (such as nitrogen, oxygen, etc) incorporation is eliminated. The as-prepared network film is just about 5-6 nm thick, but is highly resistant to chemical solvent. However, when 100 eV H+ ion bombardment is used, the aromatic rings are seriously damaged and the surface of organic film is etched. This work suggests a new route to the preparation of ultrathin polymer network films. (c) 2006 Elsevier B.V All rights reserved.