An analysis of the 1650-1350 Å band system of nitrogen dioxide has been carried out. A pattern of band spacings and intensities is found that is complex but regular. It is shown that this pattern is qualitatively, and to a large extent quantitatively, just what would be expected for a transition in which the shape of the molecule changes from bent to linear. The transition is a parallel one and the upper state has
2
Σ
+
u
symmetry. The symmetrical stretching frequency is increased from its ground-state value to
ca.
1420 cm
-1
in the upper state. The upper-state bending frequency is
ca.
600 cm
-1
. The N — O length is decreased from its groundstate value, probably to 1·1(3) Å. The upper state resembles closely the ground state of NO
+
2
. The transition is to be classed as one of the Rydberg transitions leading to the first ionization potential of NO
2
; and the orbital to which the odd electron is transferred in the transition is (pσ) in type. The anharmonic constant
g
22
for the linear upper state is found to be 2·(3) cm
-1
. Other Rydberg transitions may well be present in the region, but have not been definitely identified.
On the Nature of the First Ionization Potential of Tropolone