Benzene adsorption on Si(100)-(2 x 1) is investigated at 100 and 300 K by vibrational infrared (IR) spectroscopy, thermal desorption and near-edge X-ray absorption fine structure (NEXAFS) studies. At cryogenic temperatures (100 K), benzene is found to be predominantly physisorbed on this surface, whereas at room temperature benzene chemisorbs on Si(100)-2 x 1. IR studies show vibrational features at 2945 and 3044 cm-1 corresponding to sp3 and sp2 C-H stretching modes, respectively, and NEXAFS indicates the absence of conjugated double bonds in chemisorbed benzene. Two possible chemisorption structures are indicated, consistent with previous studies of this system. The majority structure is consistent with Diels-Alder type addition of benzene to the dimer of the Si(100)-2 x 1 surface and is described as 1,4-cyclohexadiene-like. The presence of another structure of chemisorbed benzene is suggested by a splitting in the π* transitions in the NEXAFS spectra of chemisorbed benzene. The energy difference is inconsistent with conjugation of double bonds, and the two features have different angular dependencies. The angular dependence for both π* transitions was studied and gave an angle between the double bond and the silicon surface of ∼30° for the primary 1,4-cyclohexadiene-like adduct and ∼43° for the second structure. © 1998 Elsevier Science B.V. All rights reserved.