Ion beam irradiation can be used to modify the structure and gas transport properties of glassy polymers. This is the first of two studies that focus on the impact of H+ ion irradiation on the structure and permeation properties of the polyimide Matrimid(R). Specifically, the evolution in chemical structure after H+ irradiation over a range of fluences was analyzed using FTIR spectroscopy and dissolution studies. Although H+ ion irradiation at very low ion fluences induced little modification in the chemical structure, irradiation at relatively high ion fluences resulted in crosslinking of the irradiated films. The branched structure of the aliphatic methyl (CH3) was the most sensitive to the H+ ion irradiation. The para-disubstituted aromatic ring showed the strongest resistance toward ion irradiation and required fairly high doses to induce degradation. Two potential crosslinking mechanisms related to the degradation of the aliphatic methyl and the benzophenone carbonyl were presented. (C) 2003 Wiley Periodicals, Inc.