The water sorption properties of a strongly B+ modified thin layer of a fully aromatic polyimide in humid air are studied in situ by dynamic spectroscopic ellipsometry. The polyimide structure was destroyed by the ion bombardment at ion energy of 180 keV and doses from 0.5 to 5 X 10(15) B+ /CM2. The refractive and the absorption indices of the resulted 'through-modified', carbonized layers increase with implantation dose. At the optimum implantation dose these layers swell in humid air ( > 90% rh) by approximately 2% in thickness. The swelling processes are very fast and completely reversible. (C) 2003 Elsevier B.V. All rights reserved.