The two NO(X2∏, v=1, &OHgr;=1.2,3/2) spin-orbit populations in the Earth's thermosphere have been found to depart by more than a factor of 2 from the ratio expected from thermal equilibrium. The effective temperature describing the observed population distribution is as much as 700 K lower than the local kinetic temperature. Absolute NO(v=1, J, &OHgr;) column densities were derived from high-resolution (1 cm-1) infrared earthlimb spectra for tangent altitudes up to 200 km, obtained in the CIRRIS 1A Space Shuttle experiment. Nonlinear least-squares synthetic spectral fitting was used to analyze the NO Δv=1 fundamental band emissions near 5.3 &mgr;m. The spin-orbit distribution represents a third degree of freedom, along with vibration and rotation, that is not in equilibrium with the local kinetic temperature. These observations may significantly impact the interpretation of band-integrated measurements of NO in the upper atmosphere, for which equilibrium sublevel distributions have been assumed. The subthermal distribution is most likely produced in the collisional uppumping of NO(v=0) by O atoms, which is the major source of NO(v=1) in the thermosphere. This inference suggests that the present effect is related to the subthermal spin-orbit distributions observed in laboratory studies of NO2 photodissociation. ¿ American Geophysical Union 1994