The influence of hydrogen, oxygen and lead contents on the thermo-mechanical properties of fire-refined copper has been studied and related to the decrease of its hydrogen and oxygen content by the heat treatment known as pre-heating. Fire-refined coppers contain between 0.5 and 0.7 weight ppm of hydrogen, and usually more than 50 weight ppm of lead. The decrease in the hydrogen contents of this material through pre-heating at their optimal pre-heating temperatures, leads to a strong decrease of their annealing temperature for 30% elongation (epsilon (30%)). Fire-refined coppers with a lead content higher than 20-50 weight ppm (and preferably higher than 250 weight ppm) exhibit a microstructure before pre-heating with a low number of voids, as compared to the pre-heated fire-refined copper. Pre-heating also caused a decrease of the hydrogen contents to no detectable values and a decrease of the amounts of dissolved oxygen. These results are related to the possibility that lead increases the solubility of hydrogen in copper. Pre-heating at the optimal temperature develops faceted voids and decreases the hydrogen content to undetectable values by its reaction with the dissolved oxygen, thus forming water. This theory is confirmed by the poor results of pre-heating treatment obtained for oxygen-free high-purity copper, oxygen-hydrogen-free fire-refined coppers and coppers with lead contents lower than 20-50 weight ppm.