The Green's function method and the chemisorption theory of Einstein and Schrieffer are used to calculate the chemisorption energy of O or CO on a Ni-Cu disordered binary alloy surface using the average T-matrix approximation. The change of surface segregation due to chemisorption is investigated in detail. From the chemisorption point of view, the behaviour of O resembles that of CO to a certain extent. The calculated results show that the chemisorption of O/Ni-Cu and CO/Ni-Cu systems can all constrain surface segregation, resulting in more stable chemisorption.