Interfaces between phenylacetylene (PA) monolayers and two silicon surfaces, Si(111) and Si(100). are probed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near-edge X-ray absorption One structure (NEXAFS) spectroscopy, and the results are analyzed using ab initio molecular orbital calculations. The monolayer systems are prepared via the surface hydrosilylation reaction between PA and hydrogen-terminated silicon surfaces. The following spectral features are obtained for both of the PA-Si(111) and PA -Si(100) systems: a broad pi-pi* shakeup peak at 292 eV (XPS). a broad first ionization peak at 3.8 eV (UPS), and a low-energy C Is ->pi* resonance peak at 284.3 eV (NEXAFS). These findings are ascribed to a styrene-like pi-conjugated molecular structure at the PA-Si interface by comparing the experimental data with theoretical analysis results. A conclusion is drawn that the vinyl group can keep its pi-conjugation character on the hydrogen-terminated Si(100)[H:Si(100)]surface composed of the dihydride (SiH2) groups as well as on hydrogen-terminated Si(111) having the monohydride (Sill) group. The formation mechanism of the PA-Si(100) interlace is investigated within cluster ab initio calculations, and the possible structure of the I-I:Si(100) surface is discussed based on available data.