Carbon monoxide adsorption on sulphided Mo catalysts has been investigated by means of IR spectroscopy and DFT ab initio calculations. IR experiments show that CO adsorption on the sulphide phase of Mo/Al2O3 catalysts gives rise to various v(CO) bands, the intensities of which are strongly modified when post-treatment of the catalyst with H-2 or H2S is performed before CO adsorption, therefore, revealing strong modifications in the nature and the number of sites present on the sulphide phase. Ab initio periodic DFT calculations allow to define two types of edges for MoS2, which sulphur coverage and structure depend on the H-2/HS ratio in the surrounding atmosphere. Adsorption energies and stretching wavenumber of CO adsorbed on the various sites of these surfaces were computed, providing the possibility to compare for the first time results from theoretical calculations and spectroscopic measurements on these systems. A novel attribution of the main IR features of CO adsorbed on MoS2 is proposed.