The room-temperature reaction between copper (Cu) and silicon (Si) was investigated. The areas of an Si substrate covered with very thin or island-like Cu. oxidized formed thick (>100 nm) oxide (SiO2). The areas covered with thick and nonisland-like Cu film did not. These unoxidized areas transformed into SiO2 when the side surfaces of the sample were exposed to air after sectioning for transmission electron microscope observation. The supply of oxygen was found to control this oxidation process. The presence of a Cu silicide, such as Cu3Si, was found to not necessarily be needed for oxidation. The oxidation rate estimated from observation was about 150 nm/month. Copper atoms were detected at the SiO2/Si interface and identified not as silicides but as body-center-cubic-structured Cu several atom layers thick.