The mechanism for cleaning Cu surfaces using the vapor from organic acids was studied by infrared reflection-absorption spectroscopy (IR-RAS) and by analysis of products collected using a cold-trap placed downstream of a reactor. When formic acid vapor was exposed to oxidized Cu samples heated to about 200°C for about a half an hour, the oxides were reduced. The surface seemed to have a shiny metallic appearance but no Cu-containing volatile products were collected, and the decomposition of formic acid molecules into CO2 was observed. In contrast, using acetic acid vapor, Cu-containing etching products were collected and identified as a dinuclear Cu (II) acetate complex. The vapor treatment using formic and acetic acids can be reduced Cu oxides to have metallic Cu surfaces. Furthermore, controlling temperature and choice of organic acids enabled the advantageous establishment of a control for the reduction in, and etching of, the Cu oxides.