Thev1band of the NO dimer has been studied using a long-path (84 m) low-temperature (83·5 K) gas cell and a Fourier transform infrared spectrometer. Over 440 lines of (NO)2have been measured in the 1840 to 1900 cm-1region, with a resolution of 0·008 cm-1, and assigned to perpendicular (Δka= ± 1) transitions withJandKavalues up to 35 and 16, respectively. These measurements were analysed, along with the four previous microwave transitions, using ans-reduced asymmetric rotor Hamiltonian in order to obtain accurate rotational and quartic centrifugal distortion parameters for (NO)2in its ground andv1= 1 vibrational states. As far as this experiment is concerned, the NO dimer behaves as a normal, planar, semi-rigid molecule, with no observed perturbations, only modest centrifugal distortion effects, and only small changes in parameters due to vibrational excitation. The observed intensities of (NO)2lines were used to estimate a value for the dissociation energy of the dimer ofD0= 639 ± 35 cm-1, where the indicated uncertainty depends mostly on an assumption that the vibrational transition moment of NO changes by 40% or less upon complex formation.