The room-temperature chemisorption, thermal reactivity, and reaction with atomic hydrogen were compared for 1,3-cyclohexadiene, 1,4-cyclohexadiene, and cyclohexene on Si(100)-2×1 using multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), and temperature programmed reaction/desorption techniques. For both dienes, a C=C double bond remains in the chemisorbed product, indicating that surface bonding does not involve both alkene groups in the reactant. For 1,3-cyclohexadiene, multiple chemisorption configurations are implicated, consistent with [4+2] and [2+2] cycloaddition products. The average angle between the π bond in the 1,3-cyclohexadiene adsorbate and the Si(100)-2×1 surface is 35° according to NEXAFS measurements. Upon heating, both cyclohexadienes decompose, resulting in the evolution of benzene at temperatures ranging from 400 to 700 K and release of H2 from the silicon surface at 780 K, although the behavior differs slightly for the two compounds. IR annealing studies for 1,3-cyclohexadiene confirm that decomposition begins between 400 and 500 K. Following atomic hydrogenation of the chemisorbed C6 cyclic hydrocarbons, the same vibrational spectra are obtained for cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and benzene; these spectra are also similar to that of chemisorbed cyclohexene. These similarities suggest a common hydrogenation product, whose structure is consistent with a hydrogenated [2+2] adduct, independent of whether before hydrogenation the chemisorbed hydrocarbon formed a [2+2] or [4+2] product. © 2000 American Chemical Society.