The pure electronic transition S-1<--S-0 and four van der Waals vibronic bands of phenylacetylene-Ar are measured with rotational resolution in a REMPI experiment. Here the isotropic Ar atom is attached to a molecule with two covalently bound pi -electron systems. Based on the rotational analysis we obtain accurate structural data and a new assignment of the van der Waals vibronic bands up to 43 cm(-1) excess energy. A significant shift of the Ar atom from the center of the phenyl ring towards the acetylene substituent demonstrates that the acetylenic pi -electron system contributes to the van der Waals binding.