The inhibition of copper corrosion by benzotriazole (BTAH) has been studied by scanning electrochemical microscopy as a function of time and oxygen content. Under normal atmospheric conditions the inhibiting [Cu(I)-BTA] film was formed in approximately 1 h. When the oxygen content was lowered below 15 ppm, an insulating film could not be obtained in 4 h, clearly showing that the formation of the inhibiting [Cu(I)-BTA] requires oxygen. (c) 2005 The Electrochemical Society.