From an analysis of Microwave Limb Sounder (MLS) ClO data near the stratopause where it is reasonable to assume that ClO is in photochemical equilibrium, it is found that there is a strong temperature dependence of the order of 1000 K(±200 K) including the effects of scaling and the bias uncertainties in the retrieved MLS data. Results from a three- dimensional model (extended UK Universities Global Atmospheric Modelling Programme (UGAMP) general circulation model (EUGCM)) agree well with the MLS results; we obtain a temperature dependence of 950 K in the three-dimensional model. The temperature dependencies obtained from the MLS data and the EUGCM agree quite well with results from a purely photochemical box model which yields a temperature dependence of the order of 900 K. The generally good agreement between the temperature dependence of ClO obtained from the MLS measurements, from both the photochemical model and the three- dimensional general circulation model does not suggest that there are major uncertainties in our understanding of the ClO chemistry in the upper stratosphere.