The covalent binding of acetylethyne on Si(100)-2 x I has been investigated using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The HREELS spectra of chemisorbed monolayers show the absence of the C=O, C=C, and C(sp)-H stretching modes coupled with the appearance of C=C (at 1580 cm(-1)) and C(sp(2))-H (at 3067 cm(-1)) stretching modes. This demonstrates that both of the C=O and C=-C groups of acetylethyne directly participate in binding with silicon surfaces to form C-O and C=C bonds, respectively, which is further confirmed by the XPS studies. A tetra-a-binding configuration through two [2 + 2]-like cycloaddition reactions in acetylethyne binding on Si(100) is proposed to account for the experimental observation. The cycloadduct containing a C=C double bond may be employed as an intermediate for further in situ chemical syntheses of multilayer organic thin films or surface functionalization.