Various polyimide layers [2.2-2.6 mum of hexafluoroisopropylidene bis(phthalic anhydride-oxydianiline), pyromellitic dianhydride-oxydianiline, and 3,3'-4,4'-biphenyltetracarboxylic dianhydride-p-phenylenediamine] spin-coated on silicon substrates were studied with a variable-energy positron beam in combination with a Doppler-broadened annihilation radiation technique. From the experiments, the thickness of the layers was estimated with the VEPFIT routine. These values corresponded well to the values determined from interferometry and ellipsometry. Irradiation of the polyimides with 1 x 10(15) boron ions/cm(2) at an energy of 180 keV led to a strong chemical modification of the irradiated top layer. This caused the inhibition of positronium formation in the irradiated layer, which was observed as a lowering of the annihilation line S parameter. The thickness of the modified layer was estimated to be 700-800 nm. This value did not agree with the ellipsometric measurements but corresponded to the maximum implantation depth of boron ions calculated with TRIM (Transport of Ions in Matter) code. The positron results appeared somewhat larger than the TRIM estimates. Reasons for these relations are discussed.