The rotational structure of two-photon spectra of van der Waals complexes involving an open shell diatomic molecule is analyzed in terms of the spherical tensor components of the two-photon absorption operator of the uncomplexed diatom. The formalism is used to explain the rotational structure of several band systems of the NO-Ar complex correlating with Rydberg states of the bare NO molecule. All spectra recorded suggest an N-O bond distance typical for a Rydberg state. For the C (2) Pi state, the vibrationally averaged structure is T shaped. Within the effective experimental resolution no P-type doubling could be identified. For the bands correlating with the state E (2) Sigma, a significant deviation from the T-shaped structure is found. Once the rotational contour can be simulated, a complete analysis of the vibrational structure due to the intermolecular vibrations can be performed. For the Ar-NO E state, the bending and stretching frequencies suggest a close resemblance to the cationic complex Ar-NO+. (C) 1997 American Institute of Physics.