The adsorption of organic contaminants on a hydrogen-terminated Si(111) surface was investigated using attenuated total reflection Fourier transform infrared spectroscopy. When the hydrogen-terminated Si(111) surface was exposed to dry air, the sharp Si-H monohydride peak became weaker and a broad component became visible in the lower wave-number region. Furthermore, a number of bands within the C-H stretching region were observed. The intensity of the sharp Si-H band was recovered to a certain extent and those of the broad component and the C-H bands decreased after the sample was rinsed in hexane. These results suggest that the contamination by organic adsorbates is not accompanied by a chemical bond formation.