Internal oxidized copper was tested by isothermal mechanical spectroscopy in a medium temperature range (300–600 K). Experimental results show the existence of a non-thermally activated effect at low temperature and of a relaxation peak at higher temperatures. The material microstructure was studied by combination of Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectrometry (EELS). The TEM study allowed us to investigate the distribution of fine spherical particles and the presence of particular network dislocations inside the grains. The EELS method was used to identify the nature of these fine particles as Cu2O. The internal friction has revealed a non thermally activated maximum occurring at 0.1 Hz for temperatures ranging from 290 K to 394 K, and a relaxation peak obtained after annealing at 573 K. This peak is stable after successive annealings at 723 K and 873 K. Comparison of the microstructure observations, their evolution with annealing and the evolution of the relaxation effect with annealing temperature enables us to interpret the phenomena described in this work: on the one hand, the microstructural characterisation using TEM and EELS allows us to assign the first effect to the result of a transformation of metastable Cu2O particles to CuO under the cyclic stress; on the other hand, the relaxation peak that does not change after high temperature annealing is linked with a particular stable dislocation network observed in many grains.