The physical adsorption of NO on purified single-walled carbon nanotubes was investigated at low temperatures by means of transmission infrared spectroscopy. Ozone treatment followed by annealing was used as the method of opening the entry ports for adsorption into the interior of the nanotubes. The IR spectra show that the NO that is adsorbed inside nanotubes exists exclusively in its dimer form, with no infrared bands of the monomer visible at a sensitivity level of 5 mol % NO. The internally adsorbed NO dimer species observed is cis-(NO)(2), with nu(1)(symmetric) = 1853 cm(-1) and nu(5)(asymmetric) = 1754 cm(-1). Introduction of Xe into the system leads to selective displacement of the internally bound cis-(NO)(2) dimer. A large spectroscopic red shift of the symmetric (-0.80%) and the asymmetric (-1.96%) modes for cis-(NO)(2) adsorbed inside nanotubes [relative to gas-phase cis-(NO)(2)] points to the strong interaction of the dimer with the nanotube interior. The enthalpy of dissociation of the adsorbed dimer molecule is 15.1 +/- 0.7 kJ/mol.