External infrared reflection absorption measurements were performed during plasma-enhanced chemical vapor deposition of silicon dioxide films at room temperature. The chemical bonding states of the films and their dependencies on deposition time were analyzed with high sensitivity by using p-polarized infrared light. Carbon contaminants adsorbed on the surface were oxidized into carboxyl or peroxyl functional groups with oxygen radicals in the plasma. These groups existed only near the surface and hydrolyzed rapidly after exposure to the air.