The analysis and control of interfaces between organic molecules and Si surfaces are basic issues for developing organic-inorganic hybrid devices. In this study, the adsorption of cata-condensed aromatics such as naphthalene, anthracene and pentacene on a Si(1 0 0)-2 x 1 surface was investigated by infrared reflection absorption spectroscopy in the multiple internal reflection geometry (MIR-IRAS). It was found that the adsorption configuration of naphthalene is strongly dependent on the surface coverage. In contrast, larger aromatics such as anthracene and pentacene show less surface coverage dependence. The details of the adsorption configurations are discussed on the basis of frequency calculations performed with a density functional theory. (C) 2004 Published by Elsevier B.V.