Development of our modern society requests a number of critical and strategic elements (platinum group metals, In, Ga, Ge. . .) and high value added elements (Au, Ag, Se, Te, Ni. . .) which are often concentrated in by-products during the extraction of base metals (Cu, Pb, Zn. . .). Further, recycling of end-of-life materials employed in high technology, renewable energy and transport by conventional extractive processes also leads to the concentration of such chemical elements and their compounds in metallurgical by-products and/or co-products. One of these materials, copper anode slime (CAS), derived from a copper electrolytic refining factory, was used for this study. The sample was subjected to isothermal treatment from 225 to 770 • C under air atmosphere and the reaction products were systematically analyzed by scanning electron microscopy through energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD) to investigate the thermal behavior of the treated sample. The main components of the anode slime (CuAgSe, Cu 2-x Se y S 1-y , Ag 3 AuSe 2) react with oxygen, producing mostly copper and selenium oxides as well as Ag-Au alloys as final products at temperatures higher than 500 • C. Selenium dioxide (SeO 2) is volatilized and recovered in pure state by cooling the gaseous phase, whilst copper(II) oxide, silver, gold and tellurium remain in the treatment residue.