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Two-dimensional graphene-like materials, namely MXenes, have been proposed as potential materials for various applications. In this work, the reactivity and selectivity of four MXenes ( i.e. M 2 C (M = Ti, V, Nb, Mo)) and their oxygen-functionalized forms ( i.e. O-MXenes or M 2 CO 2 ) toward gas molecules were investigated by using the plane wave-based Density Functional Theory (DFT) calculations. Small gas molecules, which are commonly found in flue gas streams, are considered herein. Our results demonstrated that MXenes are very reactive. Chemisorption is a predominant process for gas adsorption on MXenes. Simultaneously dissociative adsorption can be observed in most cases. The high reactivity of their non-functionalized surface is attractive for catalytic applications. In contrast, their reactivity is reduced, but the selectivity is improved upon oxygen functionalization. Mo 2 CO 2 and V 2 CO 2 present good selectivity toward NO molecules, while Nb 2 CO 2 and Ti 2 CO 2 show good selectivity toward NH 3 . The electronic charge properties explain the nature of the substrates and also interactions between them and the adsorbed gases. Our results indicated that O-MXenes are potential materials for gas-separation/capture, -storage, -sensing, etc. Furthermore, their structural stability and SO 2 -tolerant nature are attractive properties for using them in a wide range of applications. Our finding provides good information to narrow down the choices of materials to be tested in future experimental work.