[en] The uranium dioxide inelastic properties were studied measuring internal friction at low frequencies (of the order of 1 Hz). The work was developed in the 160 to 400 deg C temperature range. The effect of stoichiometry variation was studied oxidizing the sample with consequent change of the defect structure originally present in the non-stoichiometric uranium dioxide. The presence of a wide and irregular peak due to oxidation was observed at low temperatures. Activation energy calculations indicated the occurrence of various relaxation processes and assuming the existence of a peak between - 80 and - 70 deg C , the absolute value obtained for the activation energy (0,54 eV) is consistent with the observed values determined at medium and high frequencies for the stress induced reorientation of defects. The microstructure effect on the inelastic properties was studied for stoichiometric uranium dioxide, by varying grain size and porosity. These parameters have influence on the high temperature measurements of internal friction. The internal friction variation for temperatures higher than 340 deg C is thought to be due to grain boundary relaxation phenomena. (author)