The decomposition processes of an organic monolayer, whichwas formed on Si(111) via a Si-C covalent bond, induced by exposure to UV light irradiation or ozone, were investigated using attenuated total reflectance Fourier transform infrared spectroscopy. Exposure to both ozone and UV light resulted in a reduction of the intensities of the IR peaks corresponding to CH stretching vibration and bending scissors and the appearance of peaks corresponding to CO stretching and COH in-plane bending. The latter peaks initially increased, reached a maximum, and then decreased, indicating that the monolayer was decomposed through the formation of intermediates such as aldehyde and carboxylic acid. The monolayer was also decomposed by exposure only to UV light or ozone but more slowly as the time dependencies of the CH peaks showed. While the peaks corresponding to the CO stretching and the COH in-plane bending behaved similarly under the condition of exposure to ozone, they were not observed during decomposition induced by UV irradiation. These results show that, while the monolayer was decomposed through the formation of oxidized intermediates such as aldehyde and carboxylic acid under the condition of exposure to ozone, the decomposition of the monolayer under the condition of UV irradiation proceeded via cleavage of Si-C bonds by photogenerated electrons or holes without such oxidized intermediates. An increase of gauche defects as the decomposition proceeded was demonstrated by sum frequency generation spectroscopy.