The long wavelength 1A″ – 1A′ transition of HNO and DNO has been studied in absorption under high resolution using much greater absorption intensities than in the earlier work of Dalby. Seven new bands of HNO and six new bands of DNO have been found and analyzed. Values for [Formula: see text] have been found and are: [Formula: see text] and [Formula: see text]. Equilibrium rotational constants [Formula: see text], [Formula: see text], and [Formula: see text] have been determined for both molecules. The values for HNO give zero inertial defect but the values for DNO are less satisfactory, presumably due to resonance between pairs of levels (1, [Formula: see text], [Formula: see text]) and (0, [Formula: see text], [Formula: see text]). Equilibrium values for the geometrical parameters of the excited state have been evaluated and are: [Formula: see text] [Formula: see text], [Formula: see text].Some of the rotational lines of the (101)–(000) band of HNO are found to be slightly diffuse, the maximum observed line width being ~0.7 cm−1. This observation confirms the predissociation limit found in the emission studies of Clement and Ramsay and gives a value for the lifetime of the molecule in the excited state, viz τ ~7.6 × 10−12 sec. The predissociation is weak and the possible states causing this predissociation are discussed.