UV-Vis and Raman spectroscopy are applied under realistic reaction conditions to investigate the changes occurring to an industrial-like supported chromium oxide catalyst during the dehydrogenation of propane. Linking the catalytic activity to the simultaneously collected spectroscopic data has shown that coke is a key parameter to the activity of the catalyst. A small amount of coke improves its activity, most likely by an improved adsorption of propane on the catalyst, but the activity of the catalyst drops when the amount of coke exceeds a certain value. This point is visualized by a sharp increase in the overall absorbance of the sample as can be observed by UV-Vis spectroscopy and this information can be used as a signal for starting a regeneration cycle. (C) 2004 Elsevier Ltd. All rights reserved.