The formation of a black patina on copper in humid air containing traces of SO2 was investigated using on-line analysis of SO2 and corrosion product characterization (x-ray photoelectron spectroscopy, Fourier transform infrared absorption spectroscopy, x-ray diffraction, and quantitative analysis of sulfite). In humid air (>75% RH) with 4 to 69 ppb SO2, a dull black cuprite patina (200 to 300 nm thick) formed after 20 h exposure. However, when concentrations bf SO2 were higher, copper remained shiny, as did samples exposed at low humidity. Sulfate was the dominant sulfur species on shiny as well as on black samples. At high SO2 concentrations copper is suggested to be passivated by a layer of chemisorbed sulfite on the thin air-formed oxide film, while at low concentrations the film breaks down. Corrosion mechanisms are suggested for this active/passive transition on the copper surface. The formation of cuprite on the black samples resulted in a high corrosion rate, and an inverse correlation between SO2 concentration and corrosion was found. These results are in agreement with field studies on outdoor copper and bronze where the formation of a black cuprite patina and a high corrosion rate is observed in environments with low levels of SO2.