Radiation effects induced in thallium-based superconducting copper oxides Tl2Ba2CaCu2O8and Tl2Ba2Ca2Cu3O10irradiated by 5.8 GeV-Xe ions—have been investigated by high resolution electron microscopy (HREM). Two main points should be more particularly outlined: the inhomogeneous character of the irradiation induced-defects regions without amorphization. A significant difference in the radiation sensitivity of the two compounds is also observed in agreement with the electron transport properties modifications. The results are compared with those previously reported for YBa2Cu3O7-δsamples irradiated by 3.5 GeV-Xe ions and 5.3 GeV-Pb ions.