Two-color (2 + 1′) multiphoton ionization high-resolution threshold photoelectron spectroscopy has been applied to the Ar-NO van der Waals complex in a supersonic free jet. The resonant ionization process studied may be expressed by Ar-NO(X 2Π1/2,υ″ = 0) + 2hν1 → Ar-NO*(C 2Π,υ′) + hν2 → Ar-NO+ (X 1Σ +,υ+). Two strong anharmonic vibrational progressions have been observed in the threshold photoelectron spectra having frequencies of 79 and 94 cm-1. These vibrations have been assigned to the intermolecular bending and stretching motions of the (Ar-NO) + complex cation. From the threshold photoelectron spectra, the adiabatic ionization potential of Ar-NO has been determined as 73 869 ± 6 cm -1 (86 cm-1 higher than previously reported), and the dissociation energy of the (Ar-NO)+ cation has been calculated as 951 cm-1. The structural change of Ar-NO that occurs on photoionization has been calculated with the aid of simple Franck-Condon calculations, which suggest that the intermolecular bond distance of Ar-NO decreases by 1.03 Å, while the O-N-Ar angle decreases by 11.3°.