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Doublet bands of NbO in the near infrared region have been observed in emission with FTS techniques using an electrodeless 2450 MHz discharge as a source. Transitions involving the vibrational levels v = 0-4 of a low-lying a2Delta state of configuration sigma2delta and four additional doublet states (c2Pi, d2Deltai, e2Phi, and f2Pii) have been recorded at high resolution. Furthermore, a 2Sigma state, assigned as b2Sigma-, has been observed through the c2Pi (v = 0) --> b2Sigma- (v = 0, 1) transitions in the 1.6 &mgr;m region. Rotational analyses of the doublet bands have been performed. Most of the excited doublet states lie only slightly above or below the well-known B4Pi state. The density of states is fairly high immediately above B4Pi, and characterizations of some of the states in this region involve difficulties. A currently unanalyzed band, centered at 11 820 cm-1, has been preliminarily attributed to the A4Pi --> X4Sigma- (0, 0) transition. Nuclear hyperfine effects are barely detectable in orbitally degenerate states of the doublet manifold at Doppler-limited resolution, while the b2Sigma- state of configuration sigmadelta2 shows partly resolved magnetic hyperfine structure (b = -0.08191 cm-1). Large-scale all-electron CI calculations have been performed on doublet and quartet manifolds of NbO up to 22 000 cm-1. The calculations have been performed in two steps. In the first step, the spin-orbit effects within electronic states were not considered. An excellent overall agreement with experimental energies was obtained, except for the d2Deltai state. In the second step, all spin-orbit interactions were included. Also these calculations resulted in a good agreement with the experimental observations. The calculations also predict three hitherto unobserved low-lying states: 4Phi at 8545 cm-1, 2Gamma at 8430 cm-1 and 2Sigma+ at 9648 cm-1. Copyright 1997 Academic Press. Copyright 1997Academic Press