The dehydrogenation of isobutane over supported chromium oxide catalysts was studied by a combination of in situ UV-Vis diffuse reflectance spectroscopy and on line GC analysis. A well-defined set of experiments, based on an experimental design, was carried out to develop mathematical models, which quantitatively relate Cr-speciation and dehydrogenation activity with reaction temperature and time, support composition, gas composition and Cr loading. It will be shown that: (1) the dehydrogenation activity is proportional with the amount of in situ measured surface Cr2+/3+, and maximum for a 7.5 wt.% Cr/Al2O3 catalyst operating at 500 degrees C in 2% isobutane in N-2; and (2) Cr3+-sites are more active in alkane dehydrogenation than Cr2+-sites. This paper is the first example of the use of experimental design for deriving structure-activity relationships in the field of heterogeneous catalysis.