Diffuse reflectance infrared spectroscopy (DRIFTS) coupled to mass spectrometry (MS) is combined with the concentration modulation approach and phase sensitive detection (PSD) to follow CO methanation on a commercial alumina supported nickel catalyst. The modulation approach helps improving the sensitivity of DRIFTS under reaction conditions for adsorbed species that may be involved in the relevant reaction steps. Beside resident adsorbed CO species populating the surface of the catalyst during reaction at 200 and 300 degrees C, carbonyl species are identified at 200 degrees C that seem to indicate the sites where CO dissociation takes place. Such species are associated with sub-carbonyls or CO adsorbed on low coordinated Ni atoms. The product distribution observed during modulation of (CH4)-C-12 concentration in a (CO)-C-13/D-2 feed at 300 degrees C suggests that atomic C and H produced by CO and H-2 dissociation on Ni during methanation and C-H species may recombine to afford the methane product. However, the experiments do not help resolving whether adsorbed CO can be directly hydrogenated thus leaving open an important mechanistic issue. (C) 2015 Elsevier B.V. All rights reserved.