The cumulative double bond (C=C=C), an important intermediate in synthetic organic chemistry, was successfully prepared via the selective attachment of acetylethyne to Si(1 1 1)-7 x 7. The experimental observation of the characteristic vibrational modes and electronic structures of the C=C=C group in the surface species demonstrates the [4 + 2]-like cycloaddition occurring between the terminal O and C atoms of acetylethyne and the neighboring Si adatom-rest atom pair, consistent with the prediction of density functional theory calculations. Scanning tunneling microscopy images further reveal that the molecules selectively bind to the adjacent adatom-rest atom pairs on Si(1 1 1)-7 x 7.