High-energy ion irradiation effects on the thermally stimulated depolarization current (TSDC) behaviour and dielectric constant (epsilon')/loss factor have been investigated. Kapton-H samples of 25 mum thickness were irradiated with 50 MeV Si+ beam (fluence 2.3 x 10(12) and 1.38 x 10(13) ions/cm(2)). The TSD currents and dielectric constant/loss factor (frequency 1 and 100 kHz) have been investigated from room temperature to 250 'C, following the usual method. The TSDC spectrum of the irradiated samples reveal that the beta-peak (appearing at approximate to 80 degreesC) associated with dipolar relaxation has been largely affected due to if-radiation. At higher flu nce, beta-peak almost vanishes. The peak appearing at approximately 180 degreesC (alpha-peak) is due to space charge relaxation. The peak magnitude increases in samples irradiated with higher fluence. This is mainly due to the creation of new energy traps by irradiation. The ion irradiation results in a large increase in low temperature dielectric constant (30-70 degreesC) ascribed to the increase in water absorption capacity The dielectric constant in temperature range 70-180 degreesC is mainly governed by dipolar relaxation and space charge relaxation. It is suggested that though the space charge relaxation tends to increase epsilon', a major loss in carbonyl groups due to high-energy ion irradiation results in a decrease in epsilon'. In high temperature region (180-250 degreesC) the dominance of interfacial polarization increases the epsilon' value. The loss peaks appearing at approximately 50, 180 and 250 degreesC are representative of the various relaxation processes. The findings of the TSDC data are in conformity with the dielectric constant data. (C) 2003 Elsevier B.V. All rights reserved.