The molecular relaxation behaviour of polyolefins exposed to high-energy radiation has been investigated by dielectric loss (tan delta) analysis. Therefore, low-density polyethylene, high-density polyethylene (HDPE), and isotactic polypropylene (iPP) were gamma-irradiated in air to various absorbed doses (up to 700 kGy). All relaxation zones (alpha, beta, gamma, and delta in the order of decreasing temperature), between 25 K and melting temperature, were studied. The radiation-induced changes observed in the dielectric relaxation spectra were related to the modifications in the structural and morphological parameters attributed to exposure of the polyolefins to radiation. Wide-angle X-ray diffraction, infrared spectroscopy, and gel measurements were used to determine the radiation-induced changes in the crystalline structure, oxidative degradation, and the degree of network formation, respectively. The present study reveals high dielectric and/or relaxation sensitivity of polyolefins to gamma radiation. Disappearance of some relaxations (such as beta relaxation in HDPE and low temperature gamma and delta relaxations in iPP) is clearly observed with irradiation. For the other relaxations, besides the large changes in the relaxation intensity, radiation also induces smaller/larger changes in the distribution of relaxation times, peak position, and activation energy.