Two incidents concerning the contamination of copper-conductor surfaces associated with uninterruptible power systems (UPS) were recently observed. Valve-regulated lead/acid (VRLA) batteries used in the UPSs for backup powering were diagnosed as a possible source of the contamination. One of the VRLA batteries from the UPS unit was subjected to overcharge in the laboratory, to simulate the incident. Overcharge currents greater than 10 A, coupled with battery temperatures over 60°, can cause a VRLA battery to release significant amounts of hydrogen sulfide and sulfur dioxide. Under extreme conditions, hazardous levels of hydrogen sulfide could be generated. We conclude that the contamination was due to copper sulfide, Cu 2S, and copper oxide, Cu 2O, formed on copper surfaces as a consequence of hydrogen sulfide evolution and the high humidity inside the UPS enclosure resulting from battery venting.