We compared the deposition process of an amorphous carbon film using acetylene as a source gas with the deposition process using methane. The process was investigated by using infrared absorption spectroscopy in multiple internal reflection geometry (MIR-IRAS). The infrared spectra showed that sp(3)-hydrocarbon species was observed in both cases: on the other hand, the sp- and sp(2)-carbon and/or hydrocarbon species was clearly observed in the film deposited using acetylene. Moreover, we observed the changes in predominant adsorbed species with film thickness in both cases; these facts suggest that an amorphous carbon film grows with the structural changes of adsorbed species in both deposition processes. (C) 2009 Elsevier B.V. All rights reserved.