Nitric oxide reacts with Lewis acids (BF3, BCI3, BBt3, SnCL, S1F4, T1CI4), FIC1, or S02 to form a red species at 77 K. This species absorbs in the visible region near 500 nm and, thus, can be examined using resonance Raman spectroscopy when laser excitation lines near the absorption maximum are utilized. Both Raman and infrared spectra of the red species formed from four different isotopic forms of nitric oxide (l4N160, l5Nl60, l4NlsO, and 15NlsO) were recorded in order to ascertain the nature of the bonding. The results demonstrated that the asymmetric dimer 0=N-0=N was formed. The vibrational frequencies assigned to the normal isotopic species were iq(N=0 stretch) at 1778, v2(N=0 stretch) at 1690, iq(N-0 stretch) at 482, ^(in-plane bend) at 184, and iq(in-plane bend) at 116 cm-1. In addition, also observed were 2p4, 31/4, 4p4, 5p4, 2iq, 2p2, p3 + p4, p¡ + 2p4, 2p3 -2p4, 2p¡ -p4, p2 + p4, p2 + 2p4, p2 + 3^4, and iq + p4, as well as other overtones and combinations. In infrared spectra only bands corresponding to