The valence states of 1-D molecular assemblies of adsorbed benzophenone, acetophenone, and acetone on the Si(100)-(2 × 1)-H surface were studied with ultraviolet photoelectron spectroscopy (UPS) at room temperature. The molecular assemblies formed on the surface were checked by using in situ scanning tunneling microscopy (STM). Four major peaks at 3.8, 6.8, 9.0, and 11.2 eV relative to the Fermi level were observed in the UPS spectrum of benzophenone lines. The spectral shape of the benzophenone line is similar to that of acetophenone (except a shoulder at ∼6.8 eV instead of the intense peak) but very different from that of acetone, where the major peaks appear at 5.3, 7.6, and 9.2 eV. These suggest that the major peaks appearing for benzophenone and acetophenone lines arise from the phenyl rings of the adsorbed molecules. Comparison of the UPS spectra of benzophenone lines on the Si(100)-(2 × 1)-H surface and that of solid benzophenone clearly indicate that the electronic states of the phenyl rings assembled in 1-D remain largely decoupled from the substrate electronic states, i.e., the π orbitals of the phenyl ring have little or no interaction with the substrate. These findings reinforced the potential of such π-conjugated molecular assembly in fabricating the molecular-scale device with silicon surface as a template.