NH N 2 and NH CO van der Waals complexes have been prepared in rare gas matrices by in situ photolysis of the precursor molecules HN 3 and HNCO, and by in situ photolysis of NH 3 in rare gas matrices doped with N 2 and CO, respectively. NH CO has been detected in absorption on the A 3 ← X 3 Σ - transition. Its a → X fluorescence yield is very low, but not zero. The perturbing N 2 neighbour red shifts the NH b 1 Σ + X 3 Σ - transition by 0.07%, and the NH a 1 Δ X 3 Σ - transition by 0.25%, from their origins in pure argon matrices. Rotation of the NH moiety in argon is suppressed by the N 2 neighbour. High resolution spectra of the a 1 Δ ← X 3 Σ - transition consist of 10 closely spaced lines in argon (widths between 0.18 and 0.35 cm - 1 ), and of 6 lines in krypton. The lines are assigned to five double substitutional sites which host the NH N 2 van der Waals molecule in constant proportions of 3.6:3.2:1.2:1:1 in argon, and to three sites in krypton, where the proportions are 4.9:3.0:2.1. Each site contributes a doublet, which is due to electronic degeneracy lifting in the a 1 Δ state. The relative abundances of the sites and the splittings (ranging from 2 to 7.6 cm - 1 ) are practically unaffected by deuteration. The number of sites can be related with our previous conclusion that NH is trapped in substitutional sites of (local) O h and D 3 h symmetry. Annealing experiments reveal that NH is perturbed by N 2 only in its first coordination shell. Local modes, including site-specific modes of very low frequencies, are tentatively interpreted by means of model calculations.