Two statistical equilibrium models are compared to analyze the departure from local thermodynamic equilibrium (LTE) in the vibration-rotation bands of ozone in the middle atmosphere. The bands that emit in the 9--11 μm spectral interval depart from LTE due to chemical pumping, spontaneous emission, radiative absorption, and photochemical reaction. The diurnal variation in the &ngr;1 and &ngr;3 fundamental band source functions is determined by the diurnal changes in the rate of photolysis of ozone by solar radiation. The hot band source functions (v>2) are invariant above 85 km due to the diurnal invariance in the chemical pumping process. The magnitude of the hot-band source functions is uncertain because of the lack of understanding of the chemical pumping process. Collisional quenching of vibrationally excited ozone by atomic oxygen may reduce the magnitude of the &ngr;3 and &ngr;1 fundamental source functions by 10% near 100 km. ¿ American Geophysical Union 1990