Temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO) have been used to study the various cobalt species formed in a cobalt catalyst (Co I wt.%) supported on alumina-baria, with composition Al2O3(80 wt.%)-BaO (20 wt.%). The catalyst was synthesized by incipient-wetness impregnation of the support with a water solution of cobalt acetylacetonate and calcined at 550 degreesC for 5 h. In order to investigate the structure and morphology of the sample, XRD, BET and SEM analyses were performed. Depending on the temperature and nature of pre-treatment (reductive or oxidative) different Co species (Co3O4, Co3+, surface Co2+, CoAl2O4) were formed. Activity tests in the reduction of NO by CO were carried out in order to investigate the effectiveness of the catalyst depending on the redox pre-treatment. Highly dispersed Co3O4 particles, formed after a TPR up to I 100degreesC and successive TPO up to 550degreesC are active for the NO reduction at low temperature, N2O being the major product at 250degreesC, while N-2 formation was favored at temperature higher than 400degreesC. The as prepared sample, calcined at 550degreesC for 5 h, exhibits significant activity only at temperature > 400degreesC, giving selective formation of N-2. The nature of the active species in the supported catalyst was confirmed by studying the CO + NO reaction and NO decomposition over Co3O4 and CoO oxides as reference materials. On Co3O4 the NO reduction occurs with different selectivity to N2O/N-2, depending on the reaction temperature: at low temperature only N2O was detected, whereas at temperature >350degreesC also N-2 formation was observed. Then at temperature >450degreesC only the reaction NO --> N-2 takes place. Over COO the reaction starts at similar to400degreesC and N-2 is the only nitrogen product detected in the whole range of temperature investigated. Hundred percent of NO conversion into N-2 is reached at 600 degreesC. (C) 2004 Elsevier B.V. All rights reserved.