The rotational structure and polarization dependence of two-photon spectra of aligned ensembles of open shell diatomics is investigated in terms of the spherical tensor components of the two-photon absorption operator. The formalism allows the straightforward incorporation of state interactions and perturbations. It is applied to the two-photon spectroscopy of NO, in particular to the excitation of the Rydberg states derived from the 3p and 3d complexes. All states investigated show a nearly quadratic power dependence indicating the saturation of the ionization step. Transitions dominated by a zeroth rank tensor component (e.g., C (2) Pi-X (2) Pi or H (2) Sigma, H' (2) Pi-X (2) Pi) are insensitive to a possible angular momentum alignment in the ensemble. These transitions are ideally suited to determine degeneracy averaged observables, e.g., collision cross sections in a molecular beam scattering experiment or product velocity anisotropies in a single color photodissociation experiment. Rotational alignment data must be determined using two-photon transitions which are carried by a second rank tensor component (e.g., D (2) Sigma-X (2) Pi or F (2) Delta-X (2) Pi). (C) 1997 American Institute of Physics.