X-ray. photoelectron spectroscopy studies have shown that acetone, when used as a cleaning agent for copper in the presence of ambient light and water vapor, slowly reacts to form acetic acid. This reaction does not occur in the absence of light. This suggests that copper acts as a photocatalyst for the reaction between acetone and water vapor. The use of acetone for degreasing copper and copper containing alloys as recommended by ASTM E1078-97 is questioned in this paper.