The rate of copper corrosion originated by the action of formic acid vapors at 100% relative humidity was studied. Five formic vapor concentration levels (10, 50, 100, 200, and 300 ppm) were used. A copper corrosion rate of up to 1300 mg/m(2) d was measured for a period of 21 days using a gravimetric method. The patina layers were characterized using cathodic reduction, X-ray powder diffraction, Fourier transform infrared spectrometry, and scanning electron microscopy techniques. Some of the components identified in the corrosion-product layers were cuprite (Cu2O), cupric hydroxide hydrate [Cu(OH)(2). H2O], and copper formate hydrate [Cu(HCOO)(2). 4H(2)O]. The latter was formed by both cupric hydroxide and formic acid-cuprous ion complex mechanisms.