"Ant-nest" corrosion is a specific type of premature failure of copper tubes used in air-conditioning units causing the loss of refrigerant liquid and consequent environmental pollution. It is known that attack requires the simultaneous presence of moisture, oxygen, and a corrodent, usually formic acid. This type of corrosion occurs in thin-walled copper pipes, especially when copper is deoxidized high residual phosphorus. A study has been made of the copper corrosion products formed by the action of 1000 ppm formic acid vapor for 8 weeks exposure at 100% relative humidity and under thermal cycling. The corrosion-product layer was characterized using X-ray diffraction (XRD). The identified compounds were cuprite (Cu2O), copper hydroxide hydrate [Cu(OH)(2)center dot H2O], copper hydroxyformate [Cu(OH)(HCOO)], anhydrous copper formate [Cu(HCOO)(2)], copper formate dihydrate [Cu(HCOO)(2)center dot 2H(2)O], and copper formate tetrahydrate [Cu(HCOO)(2)center dot 4H(2)O]. Tentatively, the latter phase is described in relation with ant-nest corrosion in this paper. A kinetic study of the solid crystalline phases formed on copper has been performed using XRD patterns and the compound ratio has been compared over exposure time. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.2988059] All rights reserved.