Approximately 10% of all preliminary failures of copper tubes used in the HVAC industry are the direct result of ant-nest corrosion on a worldwide scale. This unusual form of localized corrosion has been detected in new tubes from some manufacturers, as well as early in service (i.e., less than one year). Laboratory studies have replicated service failures. The sources of the corrosive species that promote this form of attack are considered. Key words: Ant nest, formicary, pitting, copper, micro-tunnels, organic acids, formic acid, heat exchangers, air conditioning, lubricating fluids