Supported chromium oxide catalysts containing 0.5 and 4.0 wt% Cr, respectively, on a 7 wt% La2O3/ZrO2 carrier were studied by in situ EPR equipped with on-line GC during the catalyst pretreatment in flowing H-2 and the aromatization of n-octane in the range 293 < T < 823 K. Active catalysts contain almost exclusively Cr3+ in the form of Cr2O3 clusters and isolated, coordinatively unsaturated Cr3+ species on the surface. The latter appear to be less sensitive to poisoning by coke deposition and govern residual catalytic activity.