The oxidative dehydrogenation (ODH) of isobutane has been investigated on Ca-doped chromium oxide catalysts supported on gamma-Al2O3. The effect of Ca loading on the micro-structural properties of chromia catalysts was investigated by chemical analysis, X-ray diffraction (XRD), scanning electron microscopy with elemental mapping (SEM-EDX), UV-Vis diffuse reflectance spectroscopy (DRS), temperature programmed reduction (TPR), and micro-calorimetry of adsorbed NH3. Cr3+ and Cr6+ species dispersed on alumina, as well as alpha-Cr2O3 and CaCrO4 crystallites, were found on the catalysts surface. The relative amount of the chromium species depends on the Ca loading. The Cr3+/Cr6+ ratio decreases on increasing the Ca loading due to the preferred formation of bulk chromate species. The Ca loading affects the reducibility of the Cr6+ species and the acid sites strength distribution of the catalysts. The catalytic activity in the ODH reaction of isobutane is enhanced in the presence of amounts of calcium <2 wt.%, then it decreases with a further increase in the Ca content. The selectivity to isobutene follows the same trend showing a maximum (57% at 7% isobutane conversion) on the sample promoted with 2 wt.% of calcium. The reported data suggest that the activity and selectivity to isobutene are due to well-dispersed Cr6+ species on the alumina surface, whereas bulk chromates are less active and give mainly Co-x. On the basis of micro-structural and catalytic results we proposed that the alkali promoter plays different roles depending on the Ca/Cr ratio. Besides causing changes in surface acidity, calcium increases the amount of well-dispersed Cr6+ species in the range of low Ca/Cr ratios, enhancing the catalytic properties, whereas, at higher Ca/Cr ratios, it acts as a poison promoting the formation of the less active and selective bulk chromate species., (C) 2003 Elsevier B.V. All rights reserved.