A near- and far-infrared study of (NO)-N-14-(NO)-N-14 and its isotopomers, (NO)-N-14-(NO)-N-15 and (NO)-N-15-(NO)-N-15, isolated in argon matrix has been carried out. Infrared active fundamental vibrational modes, nu(1), nu(2), nu(3), nu(5), and nu(6), for each species of the dimer have been observed, as well as overtone bands 2 nu(1), 2 nu(5) and combination bands nu(1) + nu(5), nu(1) + nu(6), nu(5) + nu(6), and nu(4) + nu(5). Frequency of the nonactive v4 mode was also retrieved from combination bands. From the knowledge of the 18 available frequencies of the three isotopic species, force constants for the cis-ON-NO have been calculated. The force constants characterizing the N-N stretching mode and the ONN bending mode are found to be 0.39 mdyn/Angstrom and 0.42 mdyn Angstrom/rad(2), respectively. The potential energy distribution calculated for each vibration showed that the nu(2) and nu(3) modes are strongly coupled and that there is no pure N-N stretching mode. In the same way, the nu(1) mode could be considered as a pure NO stretching mode while the vs one is coupled to the ONN bending.