The effect of chlorine, introduced as trichloroethylene, on the CO hydrogenation activity at atmospheric pressure of an alumina-supported cobalt was studied by operand diffuse reflectance FT-IR spectroscopy (DRIFTS). The IR signal of CO(ads) could be essentially decomposed into only three bands, whether chlorine was present or not. Chlorine induced a strong and partly reversible poisoning. A major effect of chlorine on the electronic structure of the cobalt particles was evidenced here, in addition to the previously reported site blocking. The electronic density removed by chlorine from the metallic cobalt limited the weakening of the C-O bonds, which occurs through electronic back-donation from cobalt to the CO pi* anti-bonding orbital. The CO(ads) exhibiting a wave number at ca. 1881 cm(-1) were most affected by chlorine, suggesting that those were associated with the most active sites. The 1881 cm(-1) species only led to a minor dipole-dipole coupling and were thus most likely part of a mono-dimensional, or even punctual, network of CO(ads). These species could possibly be bridged CO adsorbed on edges and steps, where chlorine could preferentially adsorb. The corresponding C-O bond strength increased by about 15 +/- 8 kJ/mol in the presence of chlorine. Assuming that the rate-determining step of CO hydrogenation was C-O bond dissociation, these observations are consistent with chlorine main poisoning effects being both site blocking (i.e. competitive adsorption) and an electronic effect through the strengthening of the C-O bond that made CO(ads) less likely to dissociate (i.e. modification of the activation energy barrier). (C) 2015 Elsevier Inc. All rights reserved.