Benzonitrile and its van der Waals (vdW) complexes with argon (benzonitrile-Ar and benzonitrile-Ar-2) have been studied in supersonic jets by two-color zero-kinetic-energy (ZEKE) photoelectron spectroscopy for the first time. From the (1+1’) ZEKE photoelectron spectra obtained via the first electronic excited state (S-1), we have determined adiabatic ionization potentials as I-a(benzonitrile) = 78 490 +/- 2 cm(-1) (9.7315 +/- 0.0002 eV), I-a(benzonitrile-Ar) = 78 241 +/- 4 cm(-1) (9.7007 +/- 0.0005 eV), and I-a(benzonitrile-Ar-2) = 78 007 +/- 4 cm(-1) (9.6716 +/- 0.0005 eV). These values indicate that the decrease in ionization potential due to the complex formation (Delta I-a = -249 and -483 cm(-1), respectively) is very large compared with other monosubstituted benzenes. Eleven low-frequency vibrational modes of the bare benzonitrile cation below 1300 cm(-1) have been identified in the ZEKE photoelectron spectra. The in-plane C-C=N bending mode (33(+)) and the out-of-plane C-C-C bending mode (22(+)) have been clearly identified from the ZEKE photoelectron spectra obtained via the S-1 vibrational levels 33(1) and 22(2), respectively. Furthermore, in the benzonitrile-Ar and -Ar-2 spectra, we have observed well-resolved vibrational progressions with very low frequencies of 12 and 9 cm(-1), respectively, which may be assigned to the vdW bending mode (b(x)(+)) and the vdW symmetric bending mode (b(xx)(+)) along the substituent axis, respectively.