The adsorption of CO and O-2 molecules on Au-n (n = 2-5), Cu-n(n= 2-5), and AumCun (m= 1, 2, 3; 1 = n = 6) clusters is investigated in the framework of density functional theory. The equilibrium atomic geometry and electronic structure of each cluster are determined. It is found that Au-n (n= 2-5) clusters show strong reactivity toward CO, while they do not bind to molecular oxygen. High adsorption energy values can be obtained for odd number of Cu-n clusters for both CO and O-2 adsorptions. Cu top site is energetically preferred for the adsorption of both CO and O-2 molecules in AumCun clusters. The results of electronic density of states show an sp hybridization between Cu and O atoms in O-2 adsorbed AumCun clusters. This interaction plays a major role in their binding. The highest occupied molecular orbital and the lowest unoccupied molecular orbital energy gap values and charge density differences are presented. There are charge transfers from s and d orbital electrons of Cu to p orbitals of C and of O atoms. It is observed that O-2 adsorption on some clusters can induce half metallicity which is an ideal property for possible applications in spintronics. (C) 2015 Elsevier B.V. All rights reserved.