To gain insight into the electronic surface properties' of gamma -Mo2N(100) and the CO chemisorption, density functional calculations have been undertaken on periodic surface models using the generalized gradient approximation for the exchange-correlation functional. Structure relaxation of gamma -Mo2N(100) has been performed for different models depending on the surface stoichiometry (Mo/N ratio, mimicking the nitridation effect) or composition (O adatom,mimicking the passivation effect). The reaction gamma -Mo2N(100) + X-2(g) -> gamma -Mo2N(100)-X, with X = N and O, is calculated to be exothermic. This suggests that stoichiometric (100) surfaces containing LC-fold vacancy sites are unstable with respect to the dissociative adsorption of Nz and O-2 molecules. The chemisorption sites of CO and the associated CO vibrational frequencies have been determined for the Mo2N(100)-X/CO surfaces. On a stoichiometric (100) surface, a mu (4)-bonding mode of CO is localized on the potential energy surface with the same binding energy as the ones calculated for the an-top metallic sites (DeltaH(ads) similar to1.4 eV) while the CO binding on nitrogen adatoms is an endothermic process with an adsorption enthalpy of +0.23 eV. For a full nitrogen coverage or a nitrided (100) surface, an exothermic reaction is now calculated (DeltaH(ads) = -0.76 eV) for the formation of a chemisorbed N=C=O species. For the on-top Mo and N-surf sites, the associated CO frequencies values depend on the chemical nature of the surface rather than on the coordination number of the metallic atoms. Finally, in the reaction of gas-phase CO with surface nitrogen atoms an Eley-Rideal mechanism may be operational with an activation barrier of 1.5 eV.