The dependence of surface-enhanced infrared absorption (SEIRA) on structure and vibration modes of chemisorbates was studied in a reflection-absorption mode by employing self-assembled monolayers of 4-pyridinethiol and 4-nitrothiophenol formed on Au island films deposited on glass substrates. The present work shows that both the electromagnetic and chemical effects contribute to the total SEIRA enhancement, with the former playing a predominant role. Moreover, the bands due to different vibration modes give rise to distinctly different magnitudes of enhancement, suggesting the presence of a chemical origin in the enhanced IR absorption.